CYP27B1 and hypophosphatemic rickets: VDDR is further subclassified into four different forms including VDDR1A, VDDR1B, VDDR2A, and VDDR2B, whereas hypophosphatemic rickets consists of 15 distinct disorders which are grouped into FGF-23-dependent and FGF-23-independent hypophosphatemic rickets [7].