Fam20A has a unique and highly conserved insertion in the Gly-rich loop, which triggers the formation of two unique disulfide bonds (human Fam20A: Cys209-Cys319 and Cys211-Cys323) (121), and truncation of this insertion due to aberrant RNA splicing leads to the development of tooth enamel defects called amelogenesis imperfecta in a patient (124). Here, FAM20A is linked to amelogenesis imperfecta.