Involved in left-right asymmetry in the developing embryo. Expression is first detected in the early mid-gastrulae at the dorsal margin of the blastospore and at the anterior region of the ectoderm until the late gastrula stage. It then becomes restricted to the left anterodorsal wall of the primitive gut and to the developing left somitocoelomic system in the early neurulae stage. Expression continues on the left side in the developing preoral pit, the duct of the club-shaped gland and mouth as well as in the mesoderm at the caudal end. No expression is seen in the gland body on the right side or around the gill pores. By the mid-neurulae stage expression is detected in some cells of the central nervous system (CNS), this continues along with posterior expression in to the early larval stage. Around the rim of the mouth, expression is detected in the endoderm and ectoderm at the 36 and 48 hour larval stages. Left-sided asymmetric expression continues into the 48 hour larval stage in the preoral pit, but becomes weak or disappears in other regions. Belongs to the paired homeobox family. Bicoid subfamily.