Localized expression of in the posterior mesoderm (but not in the somites), and region-specific expression in the dorsal nerve cord, of amphioxus neurulae, later embryos and larvae. The anterior limit to expression in the nerve cord is at the level of the four/five somite boundary at the neurula stage, and stabilizes to just anterior to the first nerve cord pigment spot to form. Belongs to the Antp homeobox family.