Involved in pre-mRNA splicing. Functions in association with cyclin-dependent kinases (CDKs). May play a role in the regulation of RNA polymerase II (pol II). Inhibited by the CDK-specific inhibitor CDKN1A/p21. Interacts with POLR2A via its hyperphosphorylated C-terminal domain (CTD) (By similarity). Interacts with CDK11A, CDK11B, CDK12 and CDK13. May form a ternary complex with CDK11B and casein kinase II (CKII). Interacts with pre-mRNA-splicing factors, including at least SRSF1, SRSF2 AND SRSF7/SLU7 (By similarity). Found in nuclear intrachromatin granules clusters (IGC), also called nuclear speckles, which are storage compartments for nuclear proteins involved in mRNA processing. Contains a RS region (arginine-serine dipeptide repeat) within the C-terminal domain which is the hallmark of the SR family of splicing factors. This region probably plays a role in protein-protein interactions (By similarity). Belongs to the cyclin family. Cyclin L subfamily.