There was no evidence of hyperprolactinemia (hPRL >30 ng/ml) in the NSG-Pro mouse, and the levels of circulating hPRL closely parallel those in healthy nonpregnant women, demonstrating the physiological relevance of the new NSG-Pro model as an appropriate host for PRLR-expressing human tumors and tissues. The gene discussed is PRLR; the disease is hyperprolactinemia.