fragment-length-estimation-from-read-pairs

fragment-length-estimation-from-read-pairs

Summary

Estimate the average sequencing fragment length from paired-end ChIP-Seq reads using cross-correlation analysis of read pair coordinates. This is a critical preliminary step that informs downstream pileup extension and peak calling sensitivity in ChIP-Seq analysis.

When to use

Apply this skill when you have paired-end ChIP-Seq data (BEDPE format) and need to determine the empirical fragment length (insertion length) before peak calling. This is mandatory for paired-end ChIP-Seq workflows to ensure MACS3 correctly extends coverage tracks and calculates local bias; it is especially important when the biological fragment length is unknown or expected to deviate significantly from default assumptions.

When NOT to use

• Input is single-end ChIP-Seq data (SAM, BAM, or BED format without read-pair information); fragment length estimation requires paired-end coordinate information.
• Fragment length has already been determined by external methods (e.g., Illumina fragment analyzer) and is being supplied manually via --extsize parameter; predictd estimation is redundant.
• Sample is a control/input library rather than the ChIP library; the article states predictd should be applied only to ChIP data, not controls.

Inputs

• BEDPE file (paired-end ChIP sample, e.g., CTCF_PE_ChIP_chr22_50k.bedpe.gz)

Outputs

• Fragment length estimate (scalar, reported in base pairs)
• Cross-correlation profile (internal, used to determine d)

How to apply

Run macs3 predictd with the -f BEDPE flag on the paired-end ChIP sample to estimate the average insertion length via cross-correlation of forward and reverse reads. The subcommand analyzes the distribution of distances between paired reads and outputs a single fragment length estimate (typically reported in base pairs, e.g., ~253 bp for the CTCF_PE_ChIP_chr22_50k.bedpe.gz dataset). Record this estimated fragment length for use in subsequent macs3 callpeak and macs3 pileup steps. The estimated value should be validated against known biology (e.g., typical ChIP sonication fragment sizes) and inspected for outliers that suggest read-pair artifacts or misalignment. When fragment length is successfully estimated, proceed to peak calling without manually specifying --extsize in callpeak mode, allowing the paired-end mode to automatically handle fragment extent during pileup.