CGNPs are proposed to be the cell-of-origin for medulloblastomas associated with increased Shh pathway activity (Wechsler-Reya and Scott 1999). Consistent with this hypothesis, treatment of CGNPs in culture with exogenous Shh causes expression of many of the same proliferation markers seen in this class of medulloblastomas, including Gli1, N-myc and YAP. We wished to determine whether we could use in vitro primary CGNP cultures to mechanistically dissect the effects of YAP on proliferation and survival after irradiation. Cells prepared from post-natal day 4-5 mouse cerebella are maintained in serum free medium; addition of Shh sustains their proliferation. We transduced Shh-treated, proliferating CGNPs with retroviruses carrying GFP or YAP, then irradiated them (10 Gy) twenty-four hours later. As shown in Figure 2A, ectopic YAP expression is associated with increased CGNP proliferation as determined by cyclin D2 levels, and as we have previously reported (Fernandez et al 2009). At 20 hours post-irradiation, YAP-infected CGNPs had reduced levels of cleaved caspase 3 in comparison with GFP-infected CGNPs, and reduced numbers of apoptotic cells as determined by quantification of pyknotic nuclei (GFP: 62.5+/−0.5% vs YAP: 35.5+/−5.5%). YAP-transduced CGNPs were significantly more proliferative, as indicated by levels of cyclin D2, and by Ki67 and P-Hist3 staining (Figures 2A, B). Taken together, these results indicate that CGNP cultures recapitulate the in vivo effect of YAP expression on promoting survival and proliferation after irradiation. Moreover, they suggest that YAP-mediated radiation resistance may contribute to medulloblastoma recurrence by promoting increased tumor cell survival and proliferation.