Fam20C is a protein kinase dedicated to the phosphorylation of extracellular proteins, including caseins, and is localizes within the Golgi apparatus and also is secreted. It phosphorylates the caseins as well as several other secreted proteins such as AMELX, AMTN, ENAM, SPP1 and the small integrin-binding ligand N-linked glycoproteins SIBLINGs implicated in biomineralization. Consequently, mutations in Fam20C cause an osteosclerotic bone dysplasia in humans known as Raine syndrome and hypophosphatemic rickets. The discovery of the function of Fam20C has solved a 130-year-old mystery of the identity of the kinases that phosphorylate caseins and has shed light on several human disorders of biomineralization. In addition to its role in biomineralization, Fam20C also plays a role in lipid homeostasis, wound healing and cell migration and adhesion.