Percentage of first bases in read sequences. In read sequences with 5-15 occurrences, the first base percentage is comparable with the base composition in the genome. As the read sequence occurrence increases, the percentage of A and G goes up and comprises 100% of the first bases in sequences that occur more than 100 times. Base G is the only first base in sequences that occur greater than 160 times.
Jiang et al. Virology Journal 2011 8:194 doi:10.1186/1743-422X-8-194