otk protein expression is first detected at embryonic stage 7 in the anterior midgut primordium, cephalic furrow, and along the germ band. At stage 11, expression is observed in 15 stripes corresponding to parasegments 1-15, including external epidermal cells and internalized neuroblasts of the CNS. Within each parasegment, staining is in the posterior compartment. On the epidermis, staining comprises ventral and lateral regions but does not include the midline. Later, staining is observed in the anterior compartment. At the beginni g of germ band retraction, expression is observed in segmentally repeated patches of neuroepithelial cells and more strongly in underlying neuronal cells. It is also detected transiently in the maxillary bud. In stage 13, protein disappears from the epithelium and condenses in the ventral nerve cord. It is also observed in three discontinuous gut areas including the anterior midgut, posterior midgut and hindgut where it is restricted to the visceral mesoderm. At later stages, expression is barely detectable in mesodermal tissues but is strong in the ventral cord. Protein is present throughou the CNS on neuronal cell bodies and their axons. Staining is stronger in anterior commissures than posterior commissures. Protein is also detected on axons leaving the CNS through the segmental and intersegmental nerves, in neurons of a subset of sensory organs of the PNS, and on fasciculating axons growing toward muscle targets.