Principal component analysis (PCA) of epigenomic variation. Each dot represents one experiment, with symbols indicating the antibodies that were used (if any) and colors indicating the strains. PCA was performed on genomic coverages by dividing the genome into 90 bp bins and counting the number (per million) of forward sequence reads covering each bin. a The first two components discriminate the nucleosomal marks (identical symbols are grouped). b The next two components discriminate the strains (identical colors are grouped).