Striated muscle preferentially expressed protein kinase, also known as aortic preferentially expressed protein 1, APEG-1, SPEG and KIAA1297, is a protein that belongs to the protein kinase superfamily and CAMK Ser/Thr protein kinase family. SPEG / APEG-1 contains two fibronectin type-III domains, nine Ig-like (immunoglobulin-like) domains, two protein kinase domains. Isoform 1 of SPEG is preferentially expressed in striated muscle. Non-kinase form such as isoform 3 of SPEG is predominantly expressed in the aorta. Isoform 3 of SPEG appears to be expressed only in highly differentiated ASMC in normal vessel walls and down-regulated in dedifferentiated ASMC. Isoform 3 of SPEG may have a role in regulating the growth and differentiation of arterial smooth muscle cells. Isoform 3 of SPEG is quickly down-regulated in response to vascular injury, when ASMC cells change from a quiescent to a proliferative phenotype.