Serine / threonine-protein kinase SRPK1, also known as SFRS protein kinase 1, Serine/arginine-rich protein-specific kinase 1, SR-protein-specific kinase 1 and SRPK1, is a cytoplasm and nucleus protein which belongs to the protein kinase superfamily and CMGC Ser/Thr protein kinase family. Isoform 2 of SRPK1 is predominantly expressed in the testis but is also present at lower levels in heart, ovary, small intestine, liver, kidney, pancreas and skeletal muscle. Isoform 1 of SRPK1 is only seen in the testis, at lower levels than isoform 2. SRPK1 hyperphosphorylates RS domain-containing proteins such as SFRS1, SFRS2 and ZRSR2 on serine residues during metaphase but at lower levels during interphase. SRPK1 plays a central role in the regulatory network for splicing, controlling the intranuclear distribution of splicing factors in interphase cells and the reorganization of nuclear speckles during mitosis. SRPK1 locks onto SFRS1 to form a stable complex and processively phosphorylates the RS domain. SRPK1 appears to mediate HBV core protein phosphorylation which is a prerequisite for pregenomic RNA encapsidation into viral capsids.