In insects and vertebrates, clusters of homeobox genes (HOM genes) specify anteroposterior pattern. Two closely located homeobox genes in C. elegans, lin-39 (B. Wang and C. K, unpubl.; Clark and Horvitz, pers. comm.) and mab-5 ,contain a homeobox most similar to the Antennapedia-class. lin-39 acts in the central (Clark and Horvitz, WBG 11(2), 107 (1990)) and mab-5 in the posterior body region. We found that lin-39 and mab-5 together can specify a cell fate that is different from the fate each gene specifies on its own. Such combinatorial activity in regions of overlap greatly increases the complexity of pattern that can be generated even with a small number of genes involved. In the ventral cord, there are the twelve Pn.p cells P1 .p-P12.p.At the end of L1 ,the descendants of these cells either remain mononucleate or fuse with hyp7 .In hermaphrodites, lin-39 activity defines the "vulval equivalence group" P(3-8).p and allows the production of the vulva (Clark and Horvitz, WBG 11(2),107(1990)). In males, mab-5 defines the "preanal ganglion equivalence group" P(9-11).p and allows the production of the male copulatory apparatus. With the antibody MH27 ,which recognizes a component of septate junctions, we looked which cells fuse in lin-39 and mab-5 single and double mutants. [See Figure] In males, when acting alone (that is, in either mab-5 (-)or lin-39 (-)background) both, lin-39 (+)and mab-5 (+)can prevent cell fusion within their respective domains of function (Fig.1a). In a lin-39 (-)mab-5 (-)double mutant, where both genes are inactive, all the Pn.p cells fuse. In wildtype males, where both genes are active, one might have expected a simple active phenotype, in which all the cells in the combined lin-39 and mab-5 domain [P(3-11).p] would remain unfused. Surprisingly however, the two Pn.p cells within the region of overlap of mab-5 and lin-39 activity, P(7-8).p, adopt the alternative fate, now they fuse. In hermaphrodites, only lin-39 ,but not mab-5 ,influences the fusion decision of the Pn.p cells (Fig.lb). Thus in males, lin-39 and mab-5 together promote a fate (cell fusion), that is different from the fate promoted by either gene acting alone (remain unfused). Fusion also occurs in P1 .pand P2 .p,in which neither lin-39 nor mab-5 appear to function. Our results suggest that, by an unknown mechanism, lin-39 and mab-5 together specify the same fate that is specified if both genes are inactive (cell fusion). We can speculate that the fusion of Pn.p cells to hyp7 probably represents the "hypodermal ground state". The fusion of P(7-8).p in males limits the "preanal ganglion equivalence group" to the unfused P(9-11).p. Apparently, rather than allowing fusion of P(7-8).p by shutting off lin-39 and mab-5 ,evolution selected a combinatorial control mechanism to achieve the same goal.