CDK4 and Infertility: A Cdk2 and Cdk6 dKO also results in infertility in both sexes, while a Cdk4 and Cdk6 dKO displays the same infertility seen in a Cdk4 single KO, with total female infertility and partial male infertility [29,30,31], signifying that Cdk6 does not have a dominant phenotype in mouse fertility [41].