However, some factors may limit the usefulness of active vitamin D or vitamin D analogue therapy, such as the resultant hypercalcemia and the inappropriately high levels of CYP24A1 activity, which, besides the deactivation of 1,25(OH)2D3, also have the potential to deactivate vitamin D analogues [19]. The gene discussed is CYP24A1; the disease is hypercalcemia disease.