Prevalence of alternative splicing. (A) Shown here is the prevalence of alternative splicing (AS) versus the minimum available EST coverage for the genes. EST coverage refers to the total number of ESTs that mapped to a given gene. (% genes) The fraction of the 6400 genes for which the indicated EST coverage was minimally available; (% genes AS) the fraction of genes for which one or more alternative splice patterns were identified in dbEST; (p = x) means that only those genes associated with at least one pattern of alternative splicing that passed the binomial test at the threshold frequency x (95% C.L.) were counted when calculating % genes AS. For (p = 0), no frequency threshold was imposed, such that all genes exhibiting any degree of alternative splicing were counted. % genes AS (p = 0) increased with EST coverage and approached 100% at a coverage of 700 or more. In contrast, when a threshold frequency was imposed, the values of % genes AS varied little over a wide range of EST coverage. (B) The prevalence of intron retention (IR) events versus the minimum available EST coverage. In general, intron retention events were common for human genes, but relatively few genes exhibited them with high frequency.