Many links have been made between the cell cycle machinery and neuronal biology since the discovery of oncogenes—for example, the ras [44] and myc proteins [11], [45] are expressed in Purkinje neurons, and the cell cycle protein cdk5 is involved in dendrite formation [46] Thus the evolving idea that cell cycle pathways are utilized for parallel pathways in dividing cells and neurons is supported by the finding here that the Purkinje protein cdr2 functions in mitotic control in tumor cells. This evidence concerns the gene CDK5 and neoplasm.