Substrate peptide for C-terminal Src kinase (Csk) for use in kinase assays. Csk is a cytosolic tyrosine kinase that is closely related to the kinase Src and is a well-known negative regulator of Src family kinases. Csk has both tumour suppressor and oncogenic properties and has also been implicated in regulating blood pressure, making Csk a novel therapeutic target for hypertension disorders.