In the group of treated children with PWS, we identified one case in which serum IGF1 concentration was normal but the IGF1/IGFBP3 molar ratio was low due to a high IGFBP3 concentration (+2 SDS), one case in which serum IGF1 concentration was normal but the molar ratio was high due to a very low IGFBP3 concentration (<-2 SDS) and one case in which serum IGF1 concentration was high but the molar ratio was low, due to a high IGFBP3 concentration (Figure 4). Here, IGFBP3 is linked to Prader-Willi syndrome.