Cell-cell communication involving hormones and growth factors; cell surface receptors, their characterization and specificity; the role of G-proteins in signal transduction; intracellular signals and reversible phosphorylation mechanisms. Examples considered in detail are: cyclic AMP; protein kinase A; inositol phosphates in calcium regulation; diacylglycerol and activation of protein kinase C; calcium as a second messenger. Examples of onco gene-derived proteins in signaling