The discovery of the second-messenger functions of on serine and threonine residues. Although there is inositol 1,4,5-trisphosphate (Ins(1 ,4,5)P 3) and 1,2-dia­ general agreement that PKC plays an important role in cylglycerol (DAG), the products of receptor-stimulated the initiation and/or modulation of receptor-linked re­ phosphatidylinositol 4,5-bisphosphate (PtdIns( 4,5)P ) 2 sponses, the precise nature or molecular details of this hydrolysis, marked a turning point in the studies on the involvement remain elusive. There are several sugges­ mechanisms of mediation of functional hormone and tions of possible functions of PKC, including involve­ neurotransmitter responses. The historical background ment in modulation of ionconductance, regulation of of this discovery