// Mark and remove PCR/optical duplicates using samtools fixmate and markdup. Use when preparing alignments for variant calling or when duplicate reads would bias analysis.
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| name | bio-duplicate-handling |
| description | Mark and remove PCR/optical duplicates using samtools fixmate and markdup. Use when preparing alignments for variant calling or when duplicate reads would bias analysis. |
| tool_type | cli |
| primary_tool | samtools |
Reference examples tested with: picard 3.1+, pysam 0.22+, samtools 1.19+
Before using code patterns, verify installed versions match. If versions differ:
pip show <package> then help(module.function) to check signatures<tool> --version then <tool> --help to confirm flagsIf code throws ImportError, AttributeError, or TypeError, introspect the installed package and adapt the example to match the actual API rather than retrying.
"Remove PCR duplicates from my BAM file" → Mark or remove duplicate reads using the fixmate-sort-markdup pipeline to prevent duplicate bias in variant calling.
samtools fixmate, samtools markdup (samtools)pysam.fixmate(), pysam.markdup() (pysam)Mark and remove PCR/optical duplicates using samtools.
PCR duplicates are identical copies of the same original molecule, created during library preparation. They inflate coverage, bias allele frequencies, and create false positive variant calls. Optical duplicates are flowcell-proximity artifacts of bridge amplification (especially on patterned NovaSeq / NovaSeq X / NextSeq 1000 flowcells).
Standard samtools markdup is the right tool for some assays and actively harmful for others. The decision is assay-driven:
| Assay | Standard markdup? | Recommended approach |
|---|---|---|
| Germline WGS / WES (PCR or PCR-free) | YES | samtools markdup (PCR-free still has ~0.5% optical duplicates on patterned flowcells) |
| Somatic tumor/normal (no UMI) | YES | Same |
| Exome / target capture | YES (20-50% expected) | samtools markdup |
| ChIP-seq | MARK, do not remove | Then use peak caller's auto-dup logic (macs3 --keep-dup auto) |
| CUT&RUN / CUT&Tag | MARK, do not remove | Same |
| ATAC-seq | YES, BEFORE Tn5 +4/-5 shift | Then shift coords for footprinting |
| Bulk RNA-seq (no UMIs) | NO | Duplicates are biological at highly-expressed loci; removing them biases DE proportional to expression |
| Bulk RNA-seq (with UMIs) | NO | umi_tools dedup |
| scRNA (10x, STARsolo, drop-seq) | NO | umi_tools dedup with CB+UB tags, or rely on Cell Ranger UMI counts |
| ctDNA / liquid biopsy / deep panel (UMI) | NO | fgbio GroupReadsByUmi -> CallDuplexConsensusReads |
| Twist / IDT / Roche UMI capture | NO | fgbio or Picard UmiAwareMarkDuplicatesWithMateCigar |
| Amplicon / hotspot panel (no UMI) | NO | Every read is a "duplicate" by coordinate; markdup erases the dataset. Use samtools ampliconclip instead -- see alignment-amplicon-clipping. |
| Amplicon / hotspot panel (UMI) | NO | fgbio consensus |
| Long-read native (ONT, PacBio HiFi unamplified) | NO | No PCR step; markdup is meaningless |
| PacBio HiFi amplicon | YES (rare) | pbmarkdup |
| Ancient DNA (aDNA) | YES + mapDamage | Run markdup, then mapDamage --rescale before variant calling |
| Microbiome 16S/ITS | NO | Read counts encode community structure |
If the BAM came from 10x Cell Ranger / STARsolo and samtools markdup produces a 50-95% duplicate rate, it is the wrong tool, not a bug.
| Tool | Speed | Threading | Optical | UMI | Notes |
|---|---|---|---|---|---|
samtools markdup | Fast | Yes | Yes (-d) | Limited (--barcode-tag exact-match) | Modern production default (nf-core/sarek) |
picard MarkDuplicates | Slow | No | Yes | UmiAware variant (BETA, transcriptome bug) | GATK Best Practices reference |
biobambam2 bammarkduplicates2 | Fastest | Yes | Yes | No | Sanger / 1KGP pipelines |
samblaster | Streaming, fast | No | Optional | No | Pipe directly from aligner; no name sort |
sambamba markdup | Fast | Yes | Yes | No | Less actively maintained |
fgbio GroupReadsByUmi + CallMolecularConsensusReads | Fast | Yes | n/a | Best UMI tool | Graph-based; supports duplex |
umi_tools dedup | Slow | No | n/a | Yes (mature) | Reference for scRNA / bulk UMI |
pbmarkdup | Fast | Yes | n/a | n/a | PacBio HiFi amplicons only |
Picard UmiAwareMarkDuplicatesWithMateCigar is BETA and has known bugs on transcriptome-aligned BAMs (silently keeps duplicates). Avoid for RNA-seq UMIs.
samtools markdup default is -d 0, meaning optical-duplicate detection is disabled by default. Set explicitly per platform:
| Platform | -d value | Rationale |
|---|---|---|
| HiSeq 2000/2500 (random) | 100 | Picard historic default |
| HiSeq 3000/4000/X (patterned) | 2500 | Patterned tile size larger |
| NovaSeq 6000 (patterned) | 2500 | Same as HiSeq X |
| NovaSeq X (10B) | 2500 (or up to 12000) | Larger tiles |
| NextSeq 1000/2000 (patterned) | 2500 | ExAmp duplicates span larger pixel distances |
| MiSeq, NextSeq 500/550 | 100 | Smaller / unpatterned |
| Element AVITI, MGI / DNBseq | Custom regex | Different read-name format -- supply via --read-coords |
samtools markdup -d 2500 -t -f stats.txt input.bam marked.bam
# Count optical (SQ) vs library/PCR (LB) duplicates
samtools view -f 1024 marked.bam | grep -o 'dt:Z:[A-Z][A-Z]' | sort | uniq -c
Setting -d 2500 on a HiSeq run does no harm. Forgetting -d 2500 on NovaSeq systematically under-marks optical duplicates and overestimates library complexity.
Without --use-read-groups, multi-library BAMs systematically over-mark: independent molecules from different libraries with the same coordinates get wrongly flagged as PCR duplicates. With --use-read-groups, RG tags must also match for two reads to be a duplicate (verify availability with samtools markdup --help):
samtools markdup --use-read-groups -d 2500 -t in.bam out.bam
samtools --use-read-groups keys on RG ID; Picard's library-aware behavior keys on the LB tag (allowing dedup across multiple lanes of the same library). For multi-lane single-library BAMs, Picard MarkDuplicates with READ_NAME_REGEX is closer to canonical.
Goal: Mark PCR/optical duplicates so they can be excluded from downstream variant calling and coverage analysis.
Approach: Name-sort, add mate tags with fixmate, coordinate-sort, then run markdup. The pipeline version avoids intermediate files.
Reference (samtools 1.19+):
# 1. Sort by name (required for fixmate)
samtools sort -n -o namesort.bam input.bam
# 2. Add mate information with fixmate
samtools fixmate -m namesort.bam fixmate.bam
# 3. Sort by coordinate (required for markdup)
samtools sort -o coordsort.bam fixmate.bam
# 4. Mark duplicates
samtools markdup coordsort.bam marked.bam
# 5. Index result
samtools index marked.bam
# collate is faster than sort -n; -u/-O between piped tools skips BGZF round-trips
samtools collate -O -u input.bam tmpdir/collate | \
samtools fixmate -m -u - - | \
samtools sort -u -@ 4 -T tmpdir/sort - | \
samtools markdup -@ 4 -d 2500 -t --use-read-groups \
-f markdup_stats.txt - marked.bam
samtools index marked.bam
This is ~30% faster than sort -n | fixmate | sort | markdup on typical 30x WGS.
Critical pitfall: samtools markdup requires MS and MC tags from fixmate -m. A re-sort that loses aux tags via Python round-trip silently produces a markdup output that marks almost nothing. If duplicate counts look implausibly low, verify MC:Z: is present in the input to markdup.
Adds mate information required by markdup. Must be run on name-sorted BAM.
samtools fixmate namesorted.bam fixmate.bam
# Required for markdup to work correctly
samtools fixmate -m namesorted.bam fixmate.bam
samtools fixmate -m -@ 4 namesorted.bam fixmate.bam
samtools fixmate -r -m namesorted.bam fixmate.bam
Marks or removes duplicate alignments. Requires coordinate-sorted BAM with mate tags from fixmate.
samtools markdup input.bam marked.bam
samtools markdup -r input.bam deduped.bam
samtools markdup -s input.bam marked.bam 2> markdup_stats.txt
# Default -d 0 disables optical detection. Set per platform; see decision table above.
samtools markdup -d 2500 -t input.bam marked.bam # NovaSeq / patterned
samtools markdup -d 100 -t input.bam marked.bam # HiSeq / random
samtools markdup -@ 4 input.bam marked.bam
samtools markdup -f stats.txt input.bam marked.bam
samtools flagstat marked.bam
# Look for "duplicates" line
# Count reads with duplicate flag
samtools view -c -f 1024 marked.bam
total=$(samtools view -c marked.bam)
dups=$(samtools view -c -f 1024 marked.bam)
echo "scale=2; $dups * 100 / $total" | bc
import pysam
# Sort by name
pysam.sort('-n', '-o', 'namesort.bam', 'input.bam')
# Fixmate
pysam.fixmate('-m', 'namesort.bam', 'fixmate.bam')
# Sort by coordinate
pysam.sort('-o', 'coordsort.bam', 'fixmate.bam')
# Mark duplicates
pysam.markdup('coordsort.bam', 'marked.bam')
# Index
pysam.index('marked.bam')
import pysam
with pysam.AlignmentFile('marked.bam', 'rb') as bam:
total = 0
duplicates = 0
for read in bam:
total += 1
if read.is_duplicate:
duplicates += 1
print(f'Total: {total}')
print(f'Duplicates: {duplicates}')
print(f'Rate: {duplicates/total*100:.2f}%')
import pysam
with pysam.AlignmentFile('marked.bam', 'rb') as infile:
with pysam.AlignmentFile('nodup.bam', 'wb', header=infile.header) as outfile:
for read in infile:
if not read.is_duplicate:
outfile.write(read)
For real BAMs, always use a production marker. A naive Python implementation keyed on (chrom, pos, strand) ignores 5' position correction for soft clips, ignores library/RG, treats optical = PCR, and mis-handles supplementary alignments. The result is silently wrong duplicate marks. Use samtools markdup, Picard, or fgbio depending on assay (see decision tables above).
Some aligners can mark duplicates directly during streaming:
bwa-mem2 mem ref.fa R1.fq R2.fq | \
samblaster | \
samtools sort -o marked.bam
java -jar picard.jar MarkDuplicates \
I=input.bam \
O=marked.bam \
M=metrics.txt \
OPTICAL_DUPLICATE_PIXEL_DISTANCE=2500
For UMI libraries (10x scRNA, ctDNA panels, Twist/IDT/Roche UMI capture), naive markdup destroys information. Use UMI-aware tools:
# 10x / scRNA -- group by cell barcode + UMI
umi_tools dedup --stdin=cellranger_possorted.bam --stdout=dedup.bam \
--extract-umi-method=tag --umi-tag=UB --cell-tag=CB \
--per-cell --method=directional
# Bulk UMI / ctDNA -- consensus calling (best practice for low-VAF detection)
fgbio AnnotateBamWithUmis -i raw.bam -f umi.fastq -o annotated.bam
fgbio GroupReadsByUmi -i annotated.bam -o grouped.bam --strategy=adjacency --edits=1
fgbio CallMolecularConsensusReads -i grouped.bam -o consensus.bam --min-reads=1
# Or for duplex (xGen-Prism, NEBNext duplex):
fgbio CallDuplexConsensusReads -i grouped.bam -o duplex.bam --min-reads 1 1 0
--method=directional is the default and correct -- do not use --method=unique, which treats single-base UMI errors as different molecules. samtools markdup --barcode-tag RX (since ~1.13) does exact-match UMI grouping; adequate for IDT xGen Duplex but insufficient for single-UMI applications where 1-edit errors are common.
| Task | Command |
|---|---|
| Full workflow | sort -n | fixmate -m | sort | markdup |
| Mark duplicates | samtools markdup in.bam out.bam |
| Remove duplicates | samtools markdup -r in.bam out.bam |
| Count duplicates | samtools view -c -f 1024 marked.bam |
| View non-duplicates | samtools view -F 1024 marked.bam |
| Get stats | samtools markdup -s in.bam out.bam |
| Flag | Value | Meaning |
|---|---|---|
| 0x400 | 1024 | PCR or optical duplicate |
# View only duplicates
samtools view -f 1024 marked.bam
# View non-duplicates only
samtools view -F 1024 marked.bam
# Count non-duplicates
samtools view -c -F 1024 marked.bam
| Error | Cause | Solution |
|---|---|---|
mate not found | Input not name-sorted | Run samtools sort -n first |
no MC tag | fixmate not run with -m | Re-run fixmate with -m flag |
not coordinate sorted | Input to markdup not sorted | Run samtools sort after fixmate |
samtools markdup -r (remove duplicates) is irreversible -- the records are dropped. Default to marking, not removing; downstream tools can filter on FLAG 1024. Removing pre-emptively destroys data needed for re-running QC, library complexity estimation, or switching dedup strategies.