Progression through the cell cycle is regulated by cyclin-dependent kinases (CDKs). Plants possess a unique class of CDKs, designated B-type CDKs, but seem to lack a functional CDC25 phosphatase, which is a crucial activator of the onset of mitosis in non-plant species. Based on a striking number of functional parallels between the Arabidopsis thaliana CDKB1;1 and the Drosophila melanogaster CDC25 (string), we hypothesize that the acquisition of B-type CDKs and the disappearance of CDC25 in plants might have been associated; in these coupled events, the CDC25-controlled onset of mitosis might have been evolutionarily replaced by a B-type CDK-dominated pathway, eventually resulting in the loss of the CDC25 gene.