Distribution of correlation coefficients of alternative exon pairs in human and mouse. (A) human and (B) mouse. The x-axis is the correlation coefficient (See text) and y-axis is the number of pairs in each bin. These two histograms are very consistent regardless of the data amount. Generally, the two exons in an adjacent alternative exon pair are more likely to be positively correlated (r > 0). We can further identify two major and one minor peaks at 1, 0 and -1. A coefficient of -1 means the two alternative exons in a pair are strictly mutually exclusive, while 0 and 1 indicate independent and linked inclusion/exclusion, respectively. Thus, we divided the adjacent pairs into three groups: mutually exclusive (r ≤ -0.7, ME for short), independent (-0.2 ≤ r ≤ 0.2, IND for short) and linked (0.7 ≤ r, LNK for short). These three groups show different regulatory properties and functional importance.