Expression is vital for development; may be involved in transcriptional regulation. In embryos, muscle specific expression is required for segmental nerve b (SNb) motoneuron target recognition within ventral longitudinal muscles. Has a role in establishing and maintaining embryonic muscle attachments, adult sensory cell formation (macrochaetae) and morphogenesis of adult appendages (legs, antenna aristae and male external genitalia). Has a role in the morphogenesis of the class I dendritic neurons: selective expression of ab in class I da neurons plays a pivotal role in forming dendritic arbors, which are characteristic of the class I cells. The development of more complex arbors of class II-IV neurons depends on the absence of ab. Expressed in CNS midline cells during embryonic stages 9-13. Expression also seen in cells of the stomagastric nervous system. Segmentally repeated stripes of ectodermal expression appear at stage 11 that become uniform by stage 12 and throughout embryogenesis. Expressed at variable levels in somatic muscles from stage 16 and in all imaginal disks during larval development. Expression is seen in da neurons that grow in two-dimensional dendrites underneath the epidermis during late embryonic, larval, and pupal stages. Flies exhibit disrupts to motoneuron guidance and connectivity. Loss of ab in class I neurons results in malformation of their typical comb-like arbor patterns and generation of supernumerary branch terminals.