Cyclin-dependent kinase 2 (CDK2), a CDC2/CDK-type protein kinase, regulates cell-cycle transition from G1 to S phase. CDK2 activation requires formation of a heterodimer with cyclin A, D, or E and phosphorylation by CAK. CDK2 inhibition results from association with p21Cip1 (CDKN1A) and p27Kip1 (CDKN1B). Two alternatively spliced variants and multiple transcription initiation sites have been reported. Inhibition of CDK2 has been suggested to help prevent chemotherapy-induced alopecia in cancer patients.