Fam20C appears to be the Golgi casein kinase that is involved in phosphorylating secretory pathway proteins within S-x-E motifs. Fam20C can phosphorylate the caseins and several secreted proteins which are implicated in biomineralization, such as the small integrin-binding ligand and N-linked glycoproteins (SIBLINGs) (1). Study has revealed that FAM20C plays a direct role in facilitating osteoblast differentiation (2). Mutations in Fam20C usually cause an osteosclerotic bone dysplasia in humans known as Raine syndrome (1,3).

Reference
1. Tagliabracci, V. S., J. L. Engel, et al. (2012). Secreted kinase phosphorylates extracellular proteins that regulate biomineralization. Science 336(6085): 1150-1153. PMID: 22582013
2. Wang, X., S. Wang, et al. (2012). Inactivation of a novel FGF23 regulator, FAM20C, leads to hypophosphatemic rickets in mice. PLoS Genet 8(5): e1002708. PMID: 22615579
3. Simpson, M. A., R. Hsu, et al. (2007). Mutations in FAM20C are associated with lethal osteosclerotic bone dysplasia (Raine syndrome), highlighting a crucial molecule in bone development. Am J Hum Genet 81(5): 906-912. PMID: 17924334