• A Concentrations of endogenous insulin-like growth factor (IGF)-I in the different fractions obtained by neutral chromatography on a Superdex 200 column. Serum samples from the four siblings of family 1 were analyzed and compared to a non-related prepubertal control, as all siblings were prepubertal. The amount of IGF-I detected in the fractions corresponding to the ternary complex (TC) was markedly greater in the homozygous affected subjects (II·1 and II·3) compared to the unaffected siblings (II.2 and II.4) and the prepubertal control. No IGF-I was detected in the fractions corresponding to the binary complex (BC) or free IGF-I in any of the subjects. This is most likely due to the fact that under physiological conditions 80-85% of IGF-I circulates in the TC and after the separation procedure, the amount of IGF-I in the BC or free was below the level of detection of our IGF-I assay. These results clearly demonstrate that in the affected patients the amount of IGF-I circulating in the TC is markedly elevated. 