| name | bio-hi-c-analysis-contact-pairs |
| description | Turns Hi-C/Micro-C FASTQ into a deduplicated, filtered .pairs file with pairtools and decides whether the library worked. Covers the bwa mem -SP5M / bwa-mem2 / chromap --preset hic alignment idiom (mates mapped as independent single-end reads), pairtools parse vs parse2 and the walks-policy choice (5unique pairwise vs all for Pore-C/Micro-C concatemers), pair-type classification (keep UU and rescued UC), dedup (PCR vs optical/by-tile), select by pair_type/MAPQ/distance, restriction-fragment handling (restrict, Arima dual-enzyme, Micro-C/DNase fragment-free), and allele-specific phasing (pairtools phase to two coolers). The library-QC decision uses % long-range cis as the one-number quality metric, trans as the noise floor, orientation balance as fragment-map-free dangling-end/self-circle QC, and % duplicates as a complexity proxy. Use when processing Hi-C/Micro-C/Omni-C reads into pairs, judging library quality, handling multi-enzyme or restriction-agnostic protocols, or generating allele-specific contacts. |
| tool_type | cli |
| primary_tool | pairtools |
Version Compatibility
Reference examples tested with: pairtools 1.1+, bwa 0.7.17+ (or bwa-mem2 2.2+), chromap 0.2+, samtools 1.19+, cooler 0.10+
Before using code patterns, verify installed versions match. If versions differ:
- CLI:
<tool> --version then <tool> --help to confirm flags
pairtools defaults have shifted across releases (e.g. parse --max-molecule-size is 750 bp in 1.1.x; dedup --backend defaults to scipy). parse and parse2 report DIFFERENT positions by default - confirm --report-position before mixing outputs. If a command errors, introspect with pairtools <subcommand> --help and adapt rather than retrying.
Hi-C Contact Pairs
"Turn my Hi-C reads into clean contacts and tell me if the library worked" -> Align mates independently through ligation junctions, classify and deduplicate pairs to a 5'-canonical .pairs file, then read the cis/trans and orientation statistics to decide library quality before any matrix is built.
- CLI:
bwa mem -SP5M ref.fa R1.fq R2.fq | pairtools parse -c chrom.sizes | pairtools sort | pairtools dedup | pairtools stats
The Single Most Important Modern Insight -- The Read Count Is a Lie Until Pairs Are Classified; Library Quality Is Decided in pairtools, Not in the Aligner
A Hi-C library's usable signal is not "reads sequenced" - it is the uniquely-mapped, deduplicated, long-range cis contacts. Everything between FASTQ and the matrix exists to strip a specific artifact of proximity-ligation chemistry, and the diagnostic ratios from pairtools stats reveal whether the experiment succeeded before any compute is spent binning it. Three load-bearing consequences:
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% long-range cis is the one-number quality metric; trans is the noise floor. True crosslink-ligation contacts are overwhelmingly cis and distance-decaying. Random ligation between two unrelated molecules in solution is as likely to be trans as cis-far, so trans% is a direct readout of the spurious-ligation floor. A good in-situ human library runs cis>=1kb ~50-65%, inter-chromosomal <10%. But these numbers are genome-size dependent - a yeast or bacterial genome legitimately has higher expected trans (more inter-chromosomal volume per cis distance). Never apply a human trans threshold to a microbe.
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The ligation junction lives INSIDE the read, so a local/split aligner aligning mates independently is required. A single read often sequences through a ligation junction (locus A | locus B within one read). An end-to-end aligner soft-clips or mis-maps it and the contact is lost. bwa mem -SP5M aligns R1 and R2 as independent single-end reads (-SP skips mate rescue and pairing - proper-pair logic would destroy every long-range and trans contact) and marks the 5'-most chimeric segment primary (-5, the anchor for pairtools' 5' convention). The chimera fraction rises with read length, so on 150bp PE and on Micro-C long reads this is a large, real chunk of contacts.
-
Keep UU AND rescued UC; selecting only UU silently discards every rescued ligation. A naive select pair_type=="UU" throws away the chimeric reads pairtools successfully reconstructed (UC = combined-unique) - a meaningful fraction on long reads. The 4DN standard keeps UU and UC.
Aligner Taxonomy
| Aligner | Role | Hi-C invocation | When |
|---|
| bwa mem -SP5M | reference standard; local/split alignment reconstructs in-read junctions | bwa mem -SP5M -t N ref.fa R1 R2 | default; best inter-contig accuracy in benchmarks |
| bwa-mem2 | drop-in faster reimplementation, identical output, same flags | bwa-mem2 mem -SP5M -t N ref.fa R1 R2 | when speed matters and the larger index fits RAM |
| chromap --preset hic | ultrafast integrated align + dedup + pairs (4DN .pairs out) | chromap --preset hic -x idx -r ref.fa -1 R1 -2 R2 -o out.pairs | ~10x faster scans; trades fine walk-policy control for speed |
-SP5M, letter by letter: -S skip mate rescue; -P skip pairing (no proper-pair rescue); together -SP align mates as independent single-end reads. -5 mark the 5'-most split segment primary (anchors the 5' convention). -M is legacy compatibility only (secondary flag 256 vs supplementary 2048); pairtools handles either - never agonize over -M, never drop -SP5.
Decision Tree by Scenario
| Scenario | Recommended | Why |
|---|
| Standard in-situ/Omni-C, FASTQ -> matrix | bwa mem -SP5M -> parse (5unique) -> sort -> dedup -> stats | the 4DN/distiller default; restriction-agnostic |
| Fastest scan, control not critical | chromap --preset hic | integrated align+dedup+pairs ~10x faster |
| Multi-way contacts (Pore-C, MC-3C, Micro-C walks) | parse --walks-policy all or parse2 --expand | 5unique COLLAPSES concatemers to pairwise silently |
| Micro-C / DNase Hi-C | NO fragment map; do NOT apply a 1kb min-distance cut | sub-1kb (nucleosome ladder) is signal, not artifact |
| Arima / Hi-C 3.0 dual-enzyme, fragment-level | fragment file must encode BOTH motifs (restrict -f) | a single-enzyme digest file is silently wrong |
| Repeat-heavy genome / stringent loop anchors | raise parse --min-mapq 30 | reclassifies borderline reads M, drops repeat false anchors |
| Allele-specific / diploid folding | diploid ref + -XA suboptimal hits -> pairtools phase -> two coolers | needs the suboptimal-score gap to resolve haplotypes |
| Decide whether to sequence deeper | complexity curve from dup model / preseq lc_extrap | a bare dup% without depth is meaningless |
| Build the matrix from clean pairs | -> hic-data-io (cooler cload pairs) | binning happens after classification/dedup |
| Annotate boundary/anchor coordinates | -> genome-intervals/bed-file-basics | pairs are 1-based, half-open conventions differ |
Align: Mates as Independent Single-End Reads
bwa index ref.fa
bwa mem -SP5M -t 16 ref.fa R1.fq.gz R2.fq.gz | \
samtools view -b -@ 8 - > aligned.bam
Parse, Sort, Dedup, Select: the pairtools Core
pairtools parse -c chrom.sizes --walks-policy 5unique --min-mapq 1 \
--add-columns mapq --drop-sam aligned.bam | \
pairtools sort --nproc 8 | \
pairtools dedup --max-mismatch 3 --mark-dups \
--output-stats sample.dedup.stats | \
pairtools select '(pair_type=="UU") or (pair_type=="UC")' \
-o sample.valid.pairs.gz
Dedup MUST run on a flipped, 5'-canonical file (sort does the flip); on a non-canonical file dedup under-collapses and dup% reads falsely low. --max-molecule-size (750 bp in 1.1.x) governs single-ligation chimera rescue; --max-inter-align-gap (20 bp) sets when a coverage gap becomes a null alignment.
Library QC: the Decision This Skill Owns
pairtools stats is the canonical readout. Read it as a funnel, not a single number.
pairtools stats --bytile-dups -o sample.stats.tsv sample.valid.pairs.gz
- % long-range cis (cis>=1kb, often cis>=20kb) = signal quality. trans = noise floor (genome-size dependent).
- Orientation vs distance = fragment-map-free dangling-end/self-circle QC. Above ~1kb the four orientations FF/FR/RF/RR each converge to ~25% (random, the positive QC signal). A short-range FR (inward) spike = dangling ends / undigested / self-ligation; a short-range RF (outward) spike = self-circles / religation. The distance where orientations equalize is the minimum reliable contact distance - derive the min-distance cut from this plot, do not hardcode 1kb (Micro-C structure lives below 1kb).
- % duplicates = complexity proxy, but
--bytile-dups separates OPTICAL dups (patterned NovaSeq flowcells, same tile, adjacent coordinates) from PCR dups. Only the PCR fraction reflects library complexity; reading total dup% as over-amplification wrongly condemns a good library. A bare dup% without the depth it was measured at is meaningless - use the complexity/yield curve (preseq lc_extrap) to decide whether deeper sequencing buys uniques or duplicates.
Apply the QC-derived distance cut without a fragment map:
pairtools select '(chrom1!=chrom2) or (abs(pos2-pos1) > 1000)' \
-o sample.filtered.pairs.gz sample.valid.pairs.gz
Restriction Fragments: Opt-In, Not Default
Modern pipelines SKIP fragment filtering on purpose - the distance cut + dedup + UU/UC filter + balancing absorb the residual, and a digest file is one genome-specific place to mis-specify the enzyme. pairtools restrict -f frags.bed is opt-in for: sub-kb / restriction-fragment-resolution maps, capture-Hi-C / 4C-style fragment analysis, and bench QC where the dangling-end fraction is the digestion-efficiency readout.
cooler digest --out frags.bed chrom.sizes ref.fa DpnII
pairtools restrict -f frags.bed -o restricted.pairs.gz parsed.pairs.gz
Arima dual-enzyme has FOUR junction motifs (GATCGATC, GANTGATC, GANTANTC, GATCANTC); a single-enzyme (DpnII-only) digest file silently mis-assigns fragments. Micro-C / DNase Hi-C have NO fragment map (MNase/DNase cut sequence-nonspecifically) - any tool requiring a restriction file cannot process them, which is exactly why the restriction-agnostic pairtools path became the field default.
Allele-Specific Contacts: pairtools phase -> Two Coolers
Goal: Resolve each contact to maternal vs paternal haplotype for allele-specific 3D folding.
Approach: Align to a diploid reference reporting suboptimal hits, so each read carries its best alignment on both homologs; pairtools phase reads the gap between the two best alignment scores to tag each side resolved-hap1 / resolved-hap2 / non-resolved / multi-mapper - the score gap is what separates a genuinely uninformative read from an actual repeat.
bcftools consensus -H 1 -f ref.fa phased.vcf.gz > hap1.fa
bcftools consensus -H 2 -f ref.fa phased.vcf.gz > hap2.fa
bwa mem -SP5M ref_diploid.fa R1.fq R2.fq | \
pairtools parse -c chrom.sizes --add-columns XB,AS,XS | \
pairtools phase --phase-suffixes _hap1 _hap2 --tag-mode XB | \
pairtools sort | pairtools dedup -o phased.pairs.gz
Per-Method Failure Modes
Aligned Hi-C as a normal PE library
Trigger: plain bwa mem without -SP, or proper-pair logic. Mechanism: mate rescue forces the shotgun insert model on mates from different loci. Symptom: trans% collapses, compartments vanish, sparse map. Fix: bwa mem -SP5M, mates aligned independently.
Dropped -5
Trigger: copying a pre-2016 -SP command lacking -5. Mechanism: the 5'-most chimeric segment is not primary, so pairtools picks an inconsistent anchor. Symptom: degraded flip/dedup, smeared loops. Fix: always -SP5M (or -SP5).
walks-policy 5unique on a multi-way protocol
Trigger: Pore-C/MC-3C/Micro-C walks parsed with the default. Mechanism: 5unique reports only the two 5'-most alignments, collapsing concatemers to pairwise. Symptom: "we found few multi-contacts." Fix: --walks-policy all or parse2 --expand.
Mixing parse and parse2 outputs
Trigger: combining a parse2 (.pairs, default outer/junction-anchored) file with a parse (5'-anchored) file. Mechanism: the two report positions by different conventions. Symptom: coordinates shift by the alignment length; dedup under-collapses; loops smear. Fix: one parser/convention per project; prefer plain parse if walks are not needed.
Selecting only UU
Trigger: select pair_type=="UU". Mechanism: discards rescued chimeric (UC) pairs. Symptom: lower valid-pair yield, especially on long reads. Fix: keep (pair_type=="UU") or (pair_type=="UC").
Dedup on a non-canonical file
Trigger: dedup run before sort/flip, or on mixed parse/parse2 positions. Mechanism: duplicates are not in canonical coordinates. Symptom: dup% reads falsely low - library looks better than it is. Fix: sort (flips to 5'-canonical) before dedup.
Optical dups read as PCR dups
Trigger: total dup% on a patterned flowcell taken as library complexity. Mechanism: optical (same-tile) dups inflate apparent PCR rate. Symptom: a good library condemned as over-amplified. Fix: --bytile-dups / --output-bytile-stats; judge complexity on the PCR fraction only.
Fixed 1kb cut on Micro-C
Trigger: applying Hi-C's >1kb min-distance cut to Micro-C. Mechanism: Micro-C's nucleosome-ladder signal lives below 1kb. Symptom: the structure Micro-C exists to capture is erased. Fix: derive the cut from the orientation-vs-distance plot per library.
Quantitative Thresholds
| Threshold | Source | Rationale |
|---|
| cis>=1kb ~50-65% of nodup pairs (good in-situ human) | Dovetail/Arima QC guidance (~approx) | long-range cis is the signal; library- and genome-size dependent |
| inter-chromosomal (trans) <10% (clean), 20-30% acceptable | in-situ Hi-C practice | trans is the spurious-ligation floor; NEVER apply to small genomes |
| FF/FR/RF/RR -> ~25% each above ~1kb | random strand combination at true contacts | convergence is the fragment-map-free positive QC signal |
parse --min-mapq 1 default, raise to 30 for stringency | pairtools default | 1 drops only MAPQ-0; 30 removes repeat-driven false anchors |
dedup --max-mismatch 3 bp | pairtools default | tolerates mapping wobble; 0 over-splits, larger over-collapses complexity |
parse --max-molecule-size 750 bp (1.1.x) | pairtools default | bound on single-ligation chimera rescue; revisit for unusual size selection |
| min-distance cut ~1kb (Hi-C), derive from orientation plot | orientation-equalization distance | the cut is protocol-specific; Micro-C signal is sub-1kb |
Common Errors
| Error / symptom | Cause | Solution |
|---|
| trans% high, no compartments | aligned without -SP (proper-pair logic) | re-align bwa mem -SP5M |
| Few multi-way contacts on Pore-C/Micro-C | default --walks-policy 5unique collapsed walks | --walks-policy all / parse2 --expand |
| Loops smeared, dedup under-collapses | mixed parse/parse2 position conventions | one parser per project; sort before dedup |
| Valid-pair yield lower than expected | select kept only UU | keep UU and UC |
| dup% suspiciously low | dedup ran before sort/flip | sort to 5'-canonical first |
| dup% high on NovaSeq, complexity looks bad | optical dups counted as PCR | --bytile-dups; judge on PCR fraction |
| Micro-C structure disappears after filtering | fixed 1kb min-distance cut | derive cut from orientation-vs-distance |
| Fragment assignment wrong on Arima data | single-enzyme digest file | encode all four Arima junction motifs |
phase resolves nothing | aligned to a haploid reference | diploid ref + suboptimal (-XA) alignments |
References
- Open2C, Abdennur N, Fudenberg G, Flyamer IM, Galitsyna AA, Goloborodko A, Imakaev M, Venev SV. 2024. Pairtools: from sequencing data to chromosome contacts. PLoS Comput Biol 20(5):e1012164.
- Li H. 2013. Aligning sequence reads, clone sequences and assembly contigs with BWA-MEM. arXiv:1303.3997.
- Zhang H, Song L, Wang X, et al. 2021. Fast alignment and preprocessing of chromatin profiles with Chromap. Nat Commun 12:6566.
- Durand NC, Shamim MS, Machol I, et al. 2016. Juicer provides a one-click system for analyzing loop-resolution Hi-C experiments. Cell Syst 3:95-98.
- Servant N, Varoquaux N, Lajoie BR, et al. 2015. HiC-Pro: an optimized and flexible pipeline for Hi-C data processing. Genome Biol 16:259.
- Akgol Oksuz B, Yang L, Abraham S, et al. 2021. Systematic evaluation of chromosome conformation capture assays. Nat Methods 18:1046-1055.
- Krietenstein N, Abraham S, Venev SV, et al. 2020. Ultrastructural details of mammalian chromosome architecture (Micro-C). Mol Cell 78:554-565.
- Ramani V, Cusanovich DA, Hause RJ, et al. 2016. Mapping 3D genome architecture through in situ DNase Hi-C. Nat Protoc 11:2104-2121.
- Abdennur N, Mirny LA. 2020. Cooler: scalable storage for Hi-C data and other genomically labeled arrays. Bioinformatics 36:311-316.
- Daley T, Smith AD. 2013. Predicting the molecular complexity of sequencing libraries (preseq). Nat Methods 10:325-327.
Related Skills
- hic-data-io - Bins the deduplicated valid pairs into a .cool/.mcool matrix
- matrix-operations - Balancing and O/E that the binned pairs feed into
- hic-visualization - Render contact maps from the resulting cooler
- read-alignment/bwa-alignment - Aligner upstream; this skill adds the Hi-C
-SP5M idiom
- alignment-files/duplicate-handling - General duplicate-marking context for the pairtools dedup step
- genome-intervals/bed-file-basics - Coordinate/digest BED handling for restriction fragments and anchors
- genome-assembly/scaffolding - Same Hi-C reads used to order contigs into chromosomes
