Since children with PWS are deficient in GH and at the same time hypersensitive to GH as indicated by their increased levels of serum insulin-like growth factor-I (IGF-I) after GH treatment, compared to non-PWS patients deficient in GH, the regulation of IGF-I by IGF binding proteins may be disturbed in PWS [9,21]. The gene discussed is IGF1; the disease is Prader-Willi syndrome.