PKMYT1 (Myt1), which localizes preferentially to the endoplasmic reticulum and Golgi complex, phosphorylates CDK1 (Cdc2) on threonine 14 (T14) ( Liu et al., 1997). While the CCNA:CDK2 complex is not inactivated by PKMYT1, PKMYT1 significantly reduces histone H1 kinase activity of the CCNA:CDK1 complex as compared with the mock-treated lysate or lysate in which CDK1 was replaced with the Cdc2-AF, a mutant of CDK2 in which T14 and Y15 are replaced with A and F, respectively (Booher et al. 1997). Therefore, it is the CDK subunit and not the cyclin subunit that determines whether a CDK:cyclin complex is recognized and phosphorylated by PKMYT1 (Booher et al. 1997). In the study of Booher et al. both cyclin A (a synonym for CCNA2) and cyclin A1 (CCNA1) complex with CDK2 are mentioned, and it is not clear which CCNA isoform or if both isoforms were used. Because of a wider use and ubiquitous expression of CCNA2, CCNA2 is annotated as a CCNA set member, while CCNA2 is annotated as a set candidate.

In Drosophila intestinal stem cells and enteroblasts, Myt1 depletion promotes ectopic mitoses in a cyclin A:Cdk1-dependent manner, implicating that Myt1-mediated inhibition of cyclin A:Cdk1 is involved in coupling mitotic exit with differentiation in enteroblasts (Willms et al. 2020).