Probable transcription factor required for embryogenesis, vulval development and cell fate specification of the postembryonic mesoderm (also known as the M lineage) (PubMed:21307099). Specifically, required for the specification of sex myoblast cells and their development into the muscles that are necessary for egg-laying (PubMed:21307099). In addition, may be involved in RME GABAergic motor neuron progenitor cell fate specification (PubMed:26153233). First expressed in blastomeres in early gastrulating embryos (PubMed:21307099, PubMed:26153233). Expression continues throughout embryonic development with expression in neuronal and non-neuronal progenitors (PubMed:21307099, PubMed:26153233). During larval development, it is expressed in cells including vulval, hypodermal and intestinal cells, and is only expressed in RMH motor neurons (PubMed:21307099, PubMed:26153233). First expressed in the progenitor sex myoblasts at the 16-M cell stage of mesoderm development in hermaphrodite larvae and thereafter in descendants (PubMed:21307099). Embryonic lethal whereby embryos arrest at the three-fold stage of embryogenesis. Embryos initially develop normally until the late comma stage where a delay in the elongation process culminates in severe morphological defects. RNAi-mediated knockdown results in 98% egg-laying defective mutants with muscles lacking the egg-laying muscle specific protein egl-15. In addition, mutants also exhibit a multiple vulvae phenotype with animals having two or three vulvae. Double knockdown with let-381, sys-1, hlh-1 or fozi-1 results in no sex myoblast production.