C1 domain (also known as phorbol esters/diacylglycerol binding domain) binds an important secondary messenger diacylglycerol (DAG), as well as the analogous phorbol esters.

[1] Phorbol esters can directly stimulate protein kinase C, PKC.

Phorbol esters (such as PMA) are analogues of DAG and potent tumor promoters that cause a variety of physiological changes when administered to both cells and tissues.

The N-terminal region of PKC, known as C1, binds PMA and DAG in a phospholipid and zinc-dependent fashion.

[2] The C1 region contains one or two copies of a cysteine-rich domain, which is about 50 amino-acid residues long, and which is essential for DAG/PMA-binding.