The file: hg19.gc5Base.txt.gz is the raw data used to encode the gc5Base track on hg19. This file was produced from the hg19.2bit sequence with the kent source tree utility hgGcPercent thusly: hgGcPercent -wigOut -doGaps -file=stdout -win=5 hg18 hg19.2bit \ | gzip > hg19.gc5Base.txt.gz The format of the data is variableStep wiggle data described at: http://genome.ucsc.edu/goldenPath/help/wiggle.html with a "span" size of 5 bases. Each value is for a window size of 5 bases. Thus, possible values are: 0, 20, 40, 60, 80, 100 for a GC count of: 0, 1, 2, 3, 4, 5 respectively. Note: the chromosome coordinates given are 1-relative coordinates. The first nucleotide on a chromosome is numbered 1. Example data from chr10: variableStep chrom=chr10 span=5 60001 20 60006 60 60011 80 60016 0 ... etc ... Meaning base positions 60,001 through 60,005 have value 20, base positions 60,006 through 60,010 have value 60, etc ...