CDK3 is a member of the cyclin-dependent protein kinase family. The protein promotes entry into S phase, in part by activating members of the E2F family of transcription factors. The protein also associates with cyclin C and phosphorylates the retinoblastoma 1 protein to promote exit from G0. Recombinant human CDK3 protein, fused to His-tag at N-terminus, was expressed in E.coli.