Notch1 activity regulates cell fate decisions in the embryonic cerebellum. Pseudotyped bicistronic retroviruses expressing PLAP and full length Delta1 or the Notch1 ICD were injected into the ventricle of E9.5 embryos in utero using ultrasound backscatter microscopy, and the pups sacrificed at P21. Brains of infected animals were cryosectioned sagittally and stained for alkaline phosphatase using NBT/BCIP histochemistry. (a, d) Control experiment with retrovirus expressing alkaline phosphatase alone. (b, e) Retroviral expression of Delta1 in E9.5 cerebellar progenitors yields granule cells at P21; the inset in (e) shows sections stained by immunohistochemistry for PLAP (red) and Zic2 (green). (c, f) Retroviral expression of the Notch1 ICD in E9.5 cerebellar progenitors results in Bergmann glia at P21; inset in (f) shows sections stained for PLAP (green) and BLBP (red). Scale bars in (c, f) represent 300 μm. ICD, intracellular domain; IGL, internal granule layer; ML, molecular layer.