The regulation of germ-line development in C. elegans hermaphrodites. (a) Hermaphrodites produce first sperm cells, then oocytes. The transition is regulated in part by interaction of the FBF protein with the transcript of the fem-3 gene. When FBF is absent and the fem-3 gene is activated, fem-3 transcripts are translated and sperm are produced. When the FBF is present, fem-3 translation is blocked regardless of the level of fem-3 transcription, and oocytes are produced. (b) Mutations in the sequence PME (point mutation element) eliminate FBF binding. The mutant worms produce only sperm. The FBF protein is not drawn to scale; it spans a region much wider than the PME.