Transcription factor that binds to the promoter region of the transcription factor lin-39 to repress its expression in Q neuroblasts and thereby direct left Q neuroblast (QL) daughter cell migration (PubMed:8898225, PubMed:23784779). During postembryonic development, required for posterior-specific pattern formation (PubMed:2903796). Within the posterior body region, controls epidermal, neuronal, and mesodermal cell differentiation (PubMed:2903796, PubMed:21307099). Expressed asymmetrically in the QL neuroblast. Defective QL neuroblast daughter cell migration (PubMed:8898225). Lin-39 and mab-5 double mutants have reduced sex myoblast specific expression of sem-2 in the developing mesoderm (PubMed:21307099). Belongs to the Antp homeobox family.