PS1, a homologue of the C. elegans sel-12, a protein that plays a major role in cell fate decisions (Levitan and Greenwald, 1995; Hong and Koo, 1997), has increasingly been considered an appealing signal in fundamental neurogenic pathways. PS1, like sel-12 in C. elegans, mediates LIN-12/notch signaling. The lin-12 gene mediates multiple cell– cell interactions during uterine–vulval development. In the vulva, lin-12 is required first in the decision between the 1° and 2° vulval precursor cell fate (Sternberg, 1988; Sternberg and Horvitz, 1989), and then for proper vulval morphogenesis (Sundaram and Greenwald, 1993a; Sundaram and Greenwald, 1993b).
