The need to rely on a provocative test is based on the evidence that the simple measurement of the IGF-1 levels do not distinguish between normal and GHD subjects; in fact, a low IGF-1 is a reliable diagnostic indicator of GHD in the presence of hypopituitarism, but a normal IGF-1 does not rule out GHD (17–19). The gene discussed is IGF1; the disease is hypopituitarism.