who encodes a KH RNA binding protein that functions in muscle development.

The Drosophila who (wings held-out) gene functions during the late stages of somatic muscle development when myotubes migrate and attach to specific epidermal sites. Animals lacking who function are capable of forming multinucleate myotubes, but these cells are restricted in migration. who mutants die at the end of embryogenesis with the posterior end of their cuticles arrested over the dorsal surface. Animals that possess weak who mutations either die as pupae, or survive as adults with defects in wing position. These phenotypes indicate that who also functions during metamorphosis, when muscles are reorganized to support adult structures and behavior. These embryonic and metamorphosis defects are similar to the phenotypes produced by previously identified genes that function in either muscle development or steroid signaling pathways. who transcription occurs in muscle and muscle attachment site cells during both embryogenesis and metamorphosis, and is inducible by the steroid ecdysone at the onset of metamorphosis. who encodes a protein that contains a KH RNA binding domain. Animals that possess a mutation in a conserved loop that links predicted alpha and beta structures of this RNA binding motif lack who function. These results indicate that who plays an essential role in steroid regulation of muscle development.