Hey2/Hesr2, Hes5 and Hes1 reach their peak on postnatal day 10. Negative regulators of basic helix-loop-helix transcription factors, the Id proteins, have also been shown to be gliogenic when overexpressed in CNS progenitors, possibly through inhibiting the autoinhibition of Hes1 on its own promoter , or alternatively by antagonizing the proneural bHLH transcription factors. All four of the members of the Id family are expressed in the developing retina, and reach their peak during the period from P10 to P14 . Another group of ����pro-glial���� transcription factors, the nuclear factors I-a, -b and -x , are also expressed in the developing retina, and also reach their peak on postnatal day 10 . Thus, it appears from the array data that a coordinated gliogenic period persists, and even reaches a maximum several days after the cells have exited the mitotic cell cycle. The results of the array analysis demonstrated that several downstream effectors of the Notch pathway are expressed in FACS purified Mu�� ller glia after these cells have become postmitotic. We confirmed this using a combination of BrdU labeling and immunohistochemistry for the active form of the Notch receptor . An injection of BrdU at postnatal day 5, followed by sacrifice 2 hours later, results in BrdU incorporation in S-phase cells in the CDK inhibitor peripheral fourth to one third of the retina; at this stage in development, the progenitors have all terminally exited the cell cycle in the BAY-60-7550 PDE inhibitor central retina . The Mu�� ller glia in the central retina have already begun their differentiation, as indicated by the strong labeling for Glast , while the progenitor cells in the peripheral retina have only a low level of Glast . Thus, the BrdU and Glast labeling correlate well with the transition between the progenitor and Mu�� ller glial state. When the same sections are examined for expression of Notch-ICD, the Notch pathway is active both in the peripheral retinal progenitors , but also in the central Mu�� ller glia . The in situ hybridization at P7 also shows clear Notch mRNA expression and scattered cells in the INL express the Notch ligand, Dll1 . Hes1 immunohistochemistry and Hes5 in situ hybridization , also label a band of cells across the INL, similar to the Notch expression. Together, these data demonstrate the presence of active Notch signaling in the postmitotic Mu�� ller glia. We hypothesized that maintained Notch signaling in the postmitotic Mu�� ller glia was necessary for their differentiation and tested this hypothesis by blocking Notch with a small molecule gamma-secretase inhibitor, DAPT .