||In the nematode worm C. elegans, ray patterning and the development of the male tail involve a complex inter-play of different genes. Among them, mab-21 is involved in the specification of neuronal identity for sensory ray 6. Vertebrate homologues of mab-21 have been isolated from different species, e.g. mouse, fish and human. In this study, the Xenopus laevis homologue, XMab21l2, sharing over 90% homology with other vertebrate counterparts has been isolated. The conservation of MAB21 protein sequences suggests it has critically conserved function in the vertebrates. By analyzing the tissue expression pattern of Mab21s in chick and Xenopus coupled with in vivo knockdown of XMab21l2 in Xenopus, I have provided evidences to support this critical function of Mab21s in vertebrate neural development. Using the mouse P19 embryonal carcinoma cell with retinoic acid induction as an in vitro differentiation model, Mab21l2 was shown to be up-regulated during neural differentiation. The result suggests that Mab21l2 is one of the genetic components induced by retinoic acid. Overexpression of Mab21l2 in P19 cells resulted in increased number of differentiated neurons, more extensively fasciculated neurites and increased expression of molecular axonal marker Mapt. Mab21l2 expression also enhances the response of P19 cells to retinoic acid induced neural differentiation in a dose-dependent manner. cDNA array analyses comparing the gene expression profiles of Mab21l2 over-expressing and Mab21l2 RNAi knock-down P19 cell lines have identified Mab21l2 regulated genes relevant to diverse neural differentiation stages. The results suggest that Mab21l2 represses the expression of meosdermal genes while it promotes cell commitment to a neuroectoderm fate. Among the identified Mab21l2 regulated genes, Pax6 and Gap43 are known to be associated with neural cell specification and terminal neural differentiation, respectively. Their regulatory relationships with Mab21l2 have been explored by epistasis analyses. The results suggest that Pax6 is a tightly regulated target of Mab21l2 in the retinoic induced signaling cascade. As both retinoic acid and Pax6 are critical for neural patterning of the neural tube, the Mab21l2/Pax6 regulatory axis revealed in this study could reflect similar events occurring in the in vivo neural tube differentiation during embryonic development.