Alu repetitive elements can be inserted into mature messenger RNAs via a 
splicing-mediated process termed exonization. To understand the molecular basis 
and the regulation of the process of turning intronic Alus into new exons, we 
compiled and analyzed a data set of human exonized Alus. We revealed a mechanism 
that governs 3' splice-site selection in these exons during alternative 
splicing. On the basis of these findings, we identified mutations that activated 
the exonization of a silent intronic Alu.