This effect occurs when a very high PRL value, for example 4000 ng/mL or more, saturates both the capture and signal antibodies used in immunoradiometric and chemiluminescent assays, preventing the binding of the two antibodies in a “sandwich.” The result is an apparent PRL value that is only slightly increased, suggesting that the macroadenoma is an NFPA. The gene discussed is PRL; the disease is non-functioning pituitary adenoma.