Protein kinase Cmu (PKCmu) is a novel member of the protein kinase C (PKC) family that differs from the other isoenzymes in structural and enzymatic properties. It is characterized by the presence of a pleckstrin homology (PH) domain and an amino-terminal hydrophobic region and has substrate specificity distinct from other PKC isoforms. PKCmu is a ubiquitous PKC isotype with the highest expression in the thymus, lung and peripheral blood mononuclear cells (1). PKCmu forms a complex in vivo with a phosphatidylinositol 4-kinase and a phosphatidylinositol-4-phosphate 5-kinase. A region of PKCmu between the amino-terminal transmembrane domain and the pleckstrin homology domain is shown to be involved in the association with the lipid kinases (2