Expression of dominant negative Cdk4 and Cdk6 but not Cdk2 and Cdk3 promotes survival of primary cultured sympathetic neurons deprived of trophic support. Each data point is the mean ± SEM of three samples and is expressed relative to the number of neurons present in each well at the time of NGF withdrawal. Control viruses for each vector were generated by removal of the start codon and, in some cases, by introduction of a premature stop site.DN denotes dominant negative. The values ofp derived from Student’s t tests comparing the wild-type protein with the control-stop virus at days 1 and 2 are given. A–D, Effects of overexpression of DN Cdk2 (p > 0.05) (A), DN Cdk3 (p > 0.05) (B), DN Cdk4 (p < 0.01) (C), and DN Cdk6 (p < 0.025) (D) and respective controls on the time course of survival of sympathetic neurons after withdrawal of NGF. Similar results were obtained in at least three independent experiments.