Members of the protein kinase C (PKC) family of serine/threonine kinases play a key role in regulating the differentiation and growth of diverse cell types. PKC theta is a member of the PKC family, and its predominant expression in hematopoietic cells suggests that it may play a role in signal transduction and growth regulatory pathways unique to these cells. PKC theta is a Ca(2+)-independent PKC isoform that is selectively expressed in T lymphocytes (and muscle), and it is thought to play an important role in T cell receptor-induced activation. Among several PKC isoenzymes expressed in T cells, PKC theta is unique in being rapidly recruited to the site of TCR clustering. It has been shown that PKC theta is essential for TCR-mediated T-cell activation, but it is dispensable during TCR-dependent thymocyte development.