omics.co.in logo omics.co.in

11: Epigenomics & Peak Calling (ChIP-Seq & ATAC-Seq)

1 min read • Verified October 2026 • Peer-Reviewed Learning Resource
Tested Stacks: GATK 4.6.1 Samtools 1.21 Nextflow 24.10 Seurat v5.1 Python 3.12
Prerequisites: Basic Linux CLI & Sequencing Formats
📚 Chapter 11 of 13 0% Course Completed

Chapter 11: Epigenomics & Peak Calling (ChIP-Seq & ATAC-Seq)

While the primary DNA sequence is identical across nearly all somatic cells in an organism, epigenetic modifications dictate cell-type-specific gene expression. Epigenomics assays profile open chromatin landscapes (ATAC-seq), transcription factor binding sites, and post-translational histone modifications (ChIP-seq and CUT&RUN).

The Crucial Role of the ChIP-Seq Input Control

Sonication shearing does not break genomic DNA uniformly: open chromatin regions fragment far more easily than dense heterochromatin. Without an Input DNA control (un-immunoprecipitated crosslinked DNA), an aligner and peak caller will identify open regions as false-positive antibody binding sites. MACS3 explicitly subtracts the local Poisson background estimated from the Input control.

1. Peak Calling with MACS3

# 1. Call narrow peaks for a Transcription Factor (e.g. CTCF, FOXA1)
macs3 callpeak \
    -t chip_treatment.bam \
    -c chip_input_control.bam \
    -f BAMPE \
    -g hs \
    -n ctcf_sample \
    -q 0.01 \
    --outdir macs3_results/

# 2. Call broad peaks for repressive histone marks (e.g. H3K27me3, H3K9me3)
macs3 callpeak \
    -t h3k27me3.bam \
    -c input.bam \
    -f BAMPE \
    -g hs \
    --broad \
    --broad-cutoff 0.05 \
    -n h3k27me3_broad \
    --outdir macs3_broad_results/

2. De Novo Motif Discovery with HOMER

# Discover transcription factor consensus motifs within MACS3 peaks
findMotifsGenome.pl \
    macs3_results/ctcf_sample_peaks.narrowPeak \
    hg38 \
    homer_motifs_output/ \
    -size 200 \
    -mask
⚠️

Common Pitfalls & Troubleshooting Checklist

  • Java Heap Exhaustion (GATK / Picard): If processes terminate with java.lang.OutOfMemoryError, set your environment variable explicitly: export _JAVA_OPTIONS="-Xmx16g -Djava.io.tmpdir=/tmp".
  • Index Timestamp Mismatch: If tools complain that a BAM index is missing or out of date, regenerate with samtools index -@ 4 sample.bam.
  • Conda Channel Conflicts: Avoid ABI incompatibilities between bioconda and conda-forge by setting strict priority: conda config --set channel_priority strict.
  • Chromosome Naming Contamination: Always verify whether reference FASTA headers use chr1 or 1; divergent naming causes silent alignment drops.
Was this guide helpful to your research?
Help us maintain 100% accurate, peer-reviewed bioinformatics standards.
📝 Propose Edit on GitHub ↗