FAM20C is a member of the family with sequence similarity 20 of secreted proteins. This evolutionarily conserved protein family has three members: FAM20A, FAM20B, in addition to FAM20C. The latter gene is highly expressed in mineralized tissues, which corroborates data indicating that these proteins perform important functions in the development of the mineralized tissues. In fact loss-of-function mutations in FAM20C were associated with human lethal osteosclerotic bone dysplasia (Raine syndrome). This can be explained by the Golgi casein kinase function of FAM20C, which phosphorylates secretory pathway proteins with S-x-E motifs. These substrates are involved in biomineralization.
The FAM20C protein is 584 amino acids long and has two isoforms. The 350 amino acids at the C-terminus are highly conserved among different species and, therefore, have been named CCD (conserved C-terminal domain).