The kent program hgGcPercent will measure what you want to measure from your sequences. --Hiram hgGcPercent - Calculate GC Percentage in 20kb windows usage: hgGcPercent [options] database nibDir nibDir can be a .2bit file, a directory that contains a database.2bit file, or a directory that contains *.nib files. Loads gcPercent table with counts from sequence. options: -win=<size> - change windows size (default 20000) -noLoad - do not load mysql table - create bed file -file=<filename> - output to <filename> (stdout OK) (implies -noLoad) -chr=<chrN> - process only chrN from the nibDir -noRandom - ignore randome chromosomes from the nibDir -noDots - do not display ... progress during processing -doGaps - process gaps correctly (default: gaps are not counted as GC) -wigOut - output wiggle ascii data ready to pipe to wigEncode -overlap=N - overlap windows by N bases (default 0) -verbose=N - display details to stderr during processing -bedRegionIn=input.bed Read in a bed file for GC content in specific regions and write to bedRegionsOut -bedRegionOut=output.bed Write a bed file of GC content in specific regions from bedRegionIn example: calculate GC percent in 5 base windows using a 2bit assembly (dp2): hgGcPercent -wigOut -doGaps -win=5 -file=stdout -verbose=0 \ dp2 /cluster/data/dp2 \ | wigEncode stdin gc5Base.wig gc5Base.wib