Monogenic disorders affecting calcium and bone metabolism frequently arise from germline mutations affecting the coding region of the responsible gene and are predominantly inherited as autosomal or X-linked traits (Table 1-3).1,2 This includes autosomal dominant (e.g., Multiple Endocrine Neoplasia type 1 (MEN1) and type 2A (MEN2A)), autosomal recessive (e.g., Autoimmune Polyendocrine Syndrome type 1 (APS1)), X-linked dominant (e.g., X-linked hypophosphataemic rickets) and X-linked recessive (e.g., Dent disease) patterns of inheritance. This evidence concerns the gene RET and autoimmune polyendocrine syndrome type 1.