the dmpk gene product is a ser/thr protein kinase homologous to the mrck p21-activated kinases and the rho family of kinases. data obtained by using antibodies that detect specific isoforms of dmpk indicate that the most abundant isoform of dmpk is an 80-kda protein expressed almost exclusively in smooth, skeletal, and cardiac muscles. this kinase exists both as a membrane-associated and as a soluble form in human left ventricular samples. the different c termini of dmpk that arise from alternative splicing determine its localization to the endoplasmic reticulum, mitochondria, or cytosol in transfected cos-1 cells. among the substrates for dmpk proposed by in vitro studies are phospholemman, the dihydropyridine receptor, and the myosin phosphatase targeting subunit. however, an in vivo demonstration of the phosphorylation of these substrates by dmpk remains to be established, and a link between these substrates and the clinical manifestations of myotonic dystrophy (dm) is unclear.