Lowered basal levels of 1,25(OH)2D3 in M1‐IKO and M1/M21‐DIKO mice lead, in turn, to reduced intestinal absorption of Ca and P, which causes hypocalcemia and hypophosphatemia that promotes a rise in PTH and a reduction in FGF23 levels, and a broad Cyp27b1 null‐like skeletal phenotype. The gene discussed is FGF23; the disease is hypophosphatemia.