In addition, other factors should be taken into consideration in clinical and preclinical studies concerning calcitriol antitumoral effects in patients with BC, including different populations, VDR polymorphisms, and the status of the enzymes involved in VD3 activation, such as CYP2R1, CYP27A1, CYP27B1, and CYP24A1 [224], all this due to the presence of mutations in these enzymatic components [252] or pharmacological interactions [253]. The gene discussed is CYP27A1; the disease is breast cancer.