Alu repeats are short interspersed elements whose transposition has lead to genetic variability and heritable disorders in humans. A select subset of the nearly one million Alu sequences in human DNA actually produce new transpositions. The evolution of newly inserted Alu repeats is currently a key subject for study. Mechanisms of RNA polymerase III activity and the sequence environment into which an Alu inserts might select for transcriptional and posttranscriptional determinants of Alu transposition.