Abstract
A human Alu repeat subfamily (the PV subfamily) whose members include insertional polymorphisms is found, as predicted, to differ by five tightly linked mutations relative to another subfamily of recently inserted Alu repeats. Based on these sequence differences some of the small number of polymorphic Alus can be selected from the background of nearly one million member sequences which are fixed in the human genome. Shared patterns of mutations suggest that PV subfamily members are the progeny of several different founder sequences. The additional observation that all members ofthe PV subfamily end in a stretch of uninterrupted polyadenine residues rather than merely A-rlch sequences is evidence for post-transcriptional polyadenyiation of the presumptive RNA intermediate. The drift of polyadenine sequences toward tandemly repeated A-rich motifs suggests a biological function that may select for the fixation of dispersed Alu repeats.