Abstract
The involucrin genes of the prosimian primates and of the anthropoid primates possess nonhomologous segments of repeats located at two different sites, P and M, within the coding region. The involucrin gene of the tarsioids alone contains repeats at both sites, for it derived repeats at site P from a common ancestor of tarsioids and prosimians and a repeat at site M from a later common ancestor of tarsioids and anthropoids. After their divergence from the tarsioids, the anthropoids added many more repeats to site M and excised the older segment of repeats from site P; in contrast, the tarsioids stopped adding repeats at site M, retained the earlier segment of repeats at site P, and enlarged it. In the revision of their involucrin genes, the two lineages followed alternative routes. The mechanisms by which the revisions took place have been subject to abrupt onset or termination.