is 30 to 35% identical to other mammalian Rab proteins and includes all the canonical motifs required for guanine nucleotide binding, GTP hydrolysis, membrane association, and the conformational switch between the GTP and GDP-bound state. Rab23 is a relatively divergent Rab protein with an unusually long carboxy-terminal tail. In the mouse at embryonic day 10.5, Rab23 RNA was present at low levels in most tissues, and was present at high levels in the spinal cord, somites, limb buds, and cranial mesenchyme. In the spinal cord, Rab23 was expressed at highest levels in the dorsal half of the neural tube, although it was excluded from the roof plate. In the limb bud, it was expressed in the crescent of mesenchymal cells that are capable of responding to Shh signaling.