We treated retinas from postnatal day 8 and postnatal day 12 mice with DAPT for two days, and then sectioned the retinas and labeled them with antibodies against the progenitor marker, Ascl1, and the Mu�� ller glial marker, CyclinD3 . The sections were co-labeled with Sox9, and since this is expressed in both progenitors and Mu�� ller glia, we could control for any effects on the overall number of progenitors/glia. The use of nuclear markers for the cells Gefitinib allowed us to better quantify the effects of the DAPT. When Notch signaling was blocked with DAPT, there was a striking reduction in the number of Sox9+ cells that expressed the Mu�� ller glial marker, Ccnd3, when compared with the DMSOtreated control retinas . At the same time, the DAPT caused the induction of Ascl1 in the Sox9+ Mu�� ller glia . We also used a genetic approach to determine the effects of activating the Notch pathway on Mu�� ller glial differentiation at the stages of retinal development when the other gliogenic signals are increasing. For these experiments, we generated aPax6cre; ROSANICD mice, in which cre-mediated recombination leads to constitutively activated Notch from the intracellular domain of the Notch1 gene Previous studies using a Chx10-cre/ROSANICD line of mice demonstrated that NICD SAR131675 overexpression promotes Mu�� ller glial gene expression We therefore analyzed the mice at P0, when the gliogenic signals are not yet highly expressed, and at later postnatal ages, when these signals are increasing. Areas with active Notch signaling can be visualized with the IRES nuclear GFP reporter that is co-transcribed with the NICD , and at both ages there is an increase in the expression of glial markers; however, there is a clear difference in the level of expression of glial markers in the NICDexpressing regions between the P0 and P5 retinas. At P0, NICDexpressing progenitors express only low levels of the glial markers Cralbp and S-100 and are still mitotic , suggesting they have not yet begun differentiation as post-mitotic glial cells. By contrast, at P5, the regions of the retinas that express the NICD transgene show very intense labeling for the Mu�� ller glial markers Cralbp and S-100, with levels exceeding those of adjacent wild type Mu�� ller glia . These data support a developmental switch in the role of Notch signaling in gliogenesis in the retina . In this study, we have carried out the first genome-wide analysis of glial development. We were able to take advantage of a Hes5- GFP line of mice to selectively purify retinal progenitors and Mu�� ller glia continuously through the developmental transition.