To understand the role that this specific breakpoint has in the clinical feature of the 15q overgrowth syndrome, we evaluated the IGF1R gene expression, the IGF1R biochemical activity, serum IGF1 levels and the clinical feature of Patients 1 and 2, respectively having a 568 kb pure and a 650 kb impure (Patient 2 had also a 600 16p11.2 kb deletion, this latter known as an autism susceptibility locus (10)) chromosome 15q26.3 duplication. This evidence concerns the gene IGF1 and overgrowth syndrome.