Involved in sex determination and dosage compensation. Required for proper expression of Sxl in embryonic somatic cells. Also has an essential function in the yolk nuclei. Involved in endoderm migration and midgut formation. Homodimer. Interacts with dpn (via bHLH motif). Interacts with da (via bHLH motif). Interacts with Bap60. Localizes to all the embryonic nuclei until nuclear cycle 9, when expression ceases in the prepole cell nuclei. Associates with the somatic nuclei through cycle 10. By nuclear cycle 12, distributes uniformly in the somatic portion of the embryo and no longer associates with the nuclei. After early cycle 14 (beginning of cellularization) there is very little or no expression in the periphery of the embryo or in either the somatic or germ cells. In the yolk, accumulates at the nuclei from nuclear cycle 8 until 10-11 hours after fertilization. Expressed zygotically in embryos from 0 to 12 hours after fertilization, with a peak of expression during the 2 to 4 hour period. Homozygous embryonic female lethality. Hemizygous embryonic male lethality. Mutant embryos fail to form the midgut due to the failure of endoderm migration. Consequently, the yolk is left as an unenclosed mass that moves to ectopic locations as development progresses causing a variety of terminal phenotypes. Reported to be a member of the bZIP family but does not match diagnostic signatures for this family and has low similarity to other family members. Extended N-terminus.