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Cover The cyclin-dependent kinase inhibitory protein p18^Ink4c suppresses medulloblastoma formation induced by mutations in p53 or the sonic hedgehog receptor, Patched. Shown here is an immunohistochemical analysis of wild-type mouse cerebellum at postnatal day 5, using antibodies directed against the cyclin-dependent kinase inhibitor p27^Kip1 (red), and DAPI (blue) to stain the nuclei of proliferating granule neuron precursors in the external germinal layer (EGL) of the developing organ. p27^Kip1 (red) is expressed in post-mitotic cells within the premigratory zone of the EGL, as well as in neurons that have migrated through the Purkinje layer (lightly stained with DAPI) into the internal granule layer of the newly forming cerebellum. In contrast, Ink4c expression is absent in post-mitotic granule neurons. (For details, see Uziel et al., p. 2656.)