AbstractBackgroundThe nematode mab‐21 gene specifies sensory ray cell identity and was first isolated because of its mutant sensory ray defects. Vertebrate Mab21 orthologs have since been identified in mammals and amphibians. In this report, we characterized in detail two Mab21 orthologs in mouse, Mab21l1 and Mab21l2.MethodsWe examined the genomic organizations of Mab21 genes and used northern blot and in situ hybridizations to assay their temporal‐spatial expression pattern. Their embryonic functions were revealed by specific attenuation of Mab21 messages with antisense oligos in cultured embryos.ResultsMab21l1 and Mab21l2 have very similar protein make‐up and gene structures. Both genes were expressed in overlapping domains of actively differentiating embryonic tissues. In addition, Mab21l1 had unique expression in the lens vesicles and genital tubercle whereas Mab21l2 was expressed in the retinal epithelium and umbilical cord. Mab21l1 and Mab21l2 depleted embryos had severe defects in notochord, neural tube, organogenesis, vasculogenesis, and axial turning.ConclusionsThe findings demonstrate that both Mab21 genes are required in developing embryos for embryonic turning, formation of the notochord, neural tube, and other organ tissues. Teratology 65:70–77, 2002. © 2002 Wiley‐Liss, Inc.