The putative transcription factor Lbx1 is expressed in the mantle zone of the hindbrain and spinal cord caudal to rhombomere 1, in a specific domain of the alar plate. The Lbx1 domain overlaps with the expression domains for Tlx3 and partially with the domains for Pax2/Lim1. The ventral border of the Lbx1 domain coincides with the ventral border of the dorsalmost Serrate1 stripe in the ventricular zone. The latter borders the intermediate stripe of both Delta and Lunaticfringe expression. The Lbx1 domain contains differentiated interneurons that project into the lateral longitudinal fasciculus.