The sequences of the purine-rich-element-harboring 3′SSs deviate from the constitutive 3′SS (A) and a group of them are significantly associated with alternative splicing (B-D). A. Entropy scores of the 3′SS. n = 3847, 76, 411, 71, 35, 46, 125, 131, 99 of 3′SS for columns from left to right, respectively. *: of constitutive exons from the human genome, which are overall Py-rich (Figure 1B). The range of the Student’s t-test p values of the constitutive versus each of the other groups of splice sites containing purine-rich elements are indicated. B. Abundance of alternative splicing associated with different 3′SS sequence between the Py and 3′AG. For this graph, the percentages of alternative exons relative to the total number of exons in each group were first obtained by analysis in the UCSC Genome Browser, ranging from 33% for the aaataa to 82% for the ggaaa groups, respectively. The percentage for the group of randomly chosen exons was taken as the baseline abundance 1 (dotted line); the percentages for the other groups were all normalized to it. *: a group of exons randomly chosen from the human genome. nt(-3): nucleotide at the -3 position of 3′splice sites, which is a “y” for constitutive sites. n: any nucleotide, r: purines A or G. n = 24, 63, 169, 162, 36, 121, 38 of 3′SS for columns from left to right, respectively. The p value was obtained by hypergeometric test for the abundance of alternative splicing of the G pentamer group in the whole population of 3′ splice sites examined. C. Histogram showing the distribution of the first Gs of G tracts of 627 alternative exons within the 3′SS. The first Gs peak at -10 and -8. D. Pie-shaped distribution of G tracts of the 737 alternative exons ranging from 3 to 8 Gs in a run, with G3–5 comprising ~98%.