Fig. 5. How lin-40 regulates vulval fate specification. lin-39 Hox is a pivotal regulator in maintaining the competence of the VPCs at both the L1/L2 and later stages and therefore allowing vulval induction to occur. lin-40 MTA represses vulval fate specification by inhibiting lin-39 expression during the L2 stage and thus reducing the potential of the VPCs to be induced. By contrast, the Ras/MAPK pathway positively regulates lin-39 expression during the L3 stage (Maloof and Kenyon, 1998). Thus, lin-40 antagonizes the Ras/MAPK pathway at least partly at the level of lin-39 expression. lin-40 might also function in a lin-39-independent manner to repress downstream targets required for vulval fate specification, as indicated by a broken line.