Adjacent introns tend to be lost together. The X-axis represents the pairs or clusters of adjacent intron losses and the Y-axis represents their probability of appearance when each intron is lost independently. (A) The probability distributions of the loss of adjacent intron pairs. In a case of n lost adjacent introns, the adjacent intron pairs were counted as n - 1. (B) The probability distributions of the loss of intron clusters. The cluster was defined as one or more adjacent introns. For more details of the method, see reference . The observed patterns (marked by circles) have very small probabilities of occurrence via independent loss of each intron.