The file: hg18.gc5Base.txt.gz is the raw data used to encode
the gc5Base track on hg18.  This file was produced from the hg18.2bit
sequence with the kent source tree utility hgGcPercent thusly:

    hgGcPercent -wigOut -doGaps -file=stdout -win=5 hg18 hg18.2bit \
	| gzip > hg18.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
50001   20
50006   60
50011   80
50016   0
... etc ...

Meaning base positions 50,001 through 50,005 have value 20,
base positions 50,006 through 50,010 have value 60, etc ...

      Name                    Last modified      Size  Description
      Parent Directory                             -   
      hg18.gc5Base.txt.gz     2007-08-29 16:00  1.4G