However, with a reduction in tubular mass, both 1-α-hydroxylase and erythropoietin secretion should be decreased, and evidence shows that erythropoietin secretion remains unaffected in earlier stages of CKD when decreased calcitriol is present.105 Through the actions of FGF-23, serum phosphorus concentration is maintained at the expense of calcitriol, and the decrease in calcitriol is greater than would be predicted from the numbers of nephrons lost. The gene discussed is EPO; the disease is chronic kidney disease.