The C. elegans genome project is part of a larger effort to understand how the information encoded in its DNA specifies the biology of this small nematode worm (Brenner 1974; Wood et al. 1988). It was selected for study because it is a highly tractable experimental system. Its short generation time (3 1/2 days), large brood size (300), and its modes of reproductions (self-fertilizing hermaphrodite and cross-fertilizing males) have aided genetic analysis. More than 950 genes have already been identified through mutation (Edgley and Riddle 1990; J. Hodkin and R. Durbin, pers. comm.). Its small size (~1 mm), defined number of cells (959 somatic nuclei), and transparent body and eggshell have permitted the elucidation of the cell lineage (Sulston et al. 1983) and the reconstruction of the entire nervous system at the level of the synapse (White et al. 1986). The relatively small genome has facilitated molecular investigations, which are revealing the mechanisms of sex determination (Hodgkin 1992, 1993; Kuwabara et al. 1992; Perry et al. 1993), programmed cell death (Hengartner et al. 1992), cell determination (Sternberg and Horvitz 1991; Mello et al. 1992; Bowerman et al. 1993), and specification of the nervous system (Wadsworth and Hedgecock 1992; Garriga et al. 1993). Remarkably, many of the proteins controlling these vital functions are similar to mammalian proteins, and studies of them in C. elegans are providing insight into mammalian biology.