A key diagnostic hallmark is hypophosphatemia accompanied by low or inappropriately normal 1,25-dihydroxyvitamin D (1,25 Vit D) levels, reflecting FGF-23's inhibition of renal 1-alpha-hydroxylase (CYP27B1) despite the stimulatory effects of low phosphate [7]. The gene discussed is CYP27B1; the disease is hypophosphatemia.