CDK12(Cyclin dependent kinase 12) also known as CRKRS, CRK7 is is composed of a central CTD kinase domain, several proline-rich regions, and several amino-terminal arginine/serine (RS) motifs common to splicing factors. CDK12 is important for expression of a subset of long genes with high numbers of exons including some regulators of the DNA damage response. Depletion of CDK12 results in spontaneous DNA damage and increased sensitivity to DNA damage agents. CDK12 is ubiquitously expressed and forms a complex with cyclin K that regulates phosphorylation of the C-terminal domain (CTD) of RNA polymerase II.