Vulval development in C. elegans is dependent on the response of some of the central epidermal progenitor cells in the region of the developing vulva to a chemical signal from the gonad. Signaling from the gonad is blocked by action of the vulvaless mutant let-23 so that none of the central progenitor cells form vulval structures. In the vulvaless mutant, n300, the central progenitor cells do not form.

Which gene is likely to act earlier in the vulval developmental pathway?