||In Caenorhabditis elegans, the mab-21 gene plays an important role in cell fate specification. The mab-21 mutant displays rays 4 to 6 transformation in the male tails, which indicates that this gene is required for the choice of alternate cell fates. A previous study conducted in our lab showed that the mouse homologue Mab21l2 is expressed in various tissues in mouse embryos including the eyes, midbrain, spinal cord and limb buds, which implicates Mab21l2 as an important player in neural development. In this project, we have investigated the functions of Mab21l2 in retinal development. Using in situ hybridization, the spatial and temporal expression patterns of Mab21l2 in the retina at different embryonic and postembryonic stages (E10.5 to P10.5) are established. It was found that Mab21l2 is expressed in specific retinal cell types, including retinal ganglion, amacrine and horizontal cells. Thus, Mab21l2 may be required for specification or differentiation of these cells, a notion consistent with previous reports in zebrafish. Moreover, since retinal cell types can be broadly divided into early and late cell types, while retinal ganglion, amacrine and horizontal cells belong to the early types, our results suggest that Mab21l2 functions in this early group. In a transgenic approach to mis-express Mab21l2 in non-expressing cells, bipolar and rod cells were also attempted, to evaluate its biological impacts on cell type specification. However, no cell type transformation was observed through immuno-staining. Moreover, the mis-expression of Mab21l2 was not able to induce Pax6 expression, as suggested by previous in vitro experiments. The lack of transformation may be due to an improper developmental window, low expression level or the requirement of additional factors for proper cell differentiation. Further experiments would be needed to evaluate the function of Mab21l2 in murine retina development.