PCP-4 (Purkinje cell protein 4), also known as PEP-19, is a calmodulin (CaM) regulatory protein that is highly expressed in neuronal cells. Through its IQ motif, PCP-4 mediates both the calcium-dependent binding properties of CaM and the rates of association and dissociation of calcium from the C-terminal domain of CaM. The IQ motif contains a serine residue which can be phosphorylated by all isoforms of protein kinase C (PKC). PCP-4 is implicated in uterine leiomyomas, as well as in neurodegenerative disorders such as Alzheimer’s disease and Huntington's disease. Additionally, overexpression of PCP-4 is thought to play a role in cerebellar hypoplasia, a key feature of Down syndrome.