POMC and classic congenital adrenal hyperplasia due to 21-hydroxylase deficiency: There is some evidence that ABCC1 also regulates the HPA axis in humans, potentially due to high expression in the pituitary gland, which might reduce the sensitivity of ACTH secretion to corticosterone.33 However, in support of the latter explanation, previous data from subcutaneous hydrocortisone infusions demonstrate that similar cortisol concentrations successfully suppress ACTH in patients with 21-hydroxylase deficiency.16,38