CKI delta is a member of the casein kinase 1 (CK1) serine/threonine kinase family of proteins. In mammals, there are seven distinct genes that represent the CK1 family: alpha, beta, gamma-1, gamma-2, gamma-3, delta, and epsilon. CKI delta phosphorylates a number of proteins some of which include the circadian proteins period 1 and period 2. CKI delta is also involved in the Wnt signaling pathway.