While mother, father and brothers had normal karyotypes, the patient had a 49,XXXXY chromosome constitution (Figure 1). We inferred that the polysomy appeared to be due to de novo non disjunction of the X-chromosomes. After evaluation of nine regions specific to X and Y chromosomes, the origin of the extra chromosomes was revealed. Comparison of the area of the peaks (Figure 2) for the patient and his parents showed an imbalance between his X and Y chromosome in the AMXY and DXYS218 regions (ratios 4:1), indicating his tetra X constitution. All X chromosome-specific STR markers of the patient were inherited from his mother and heterozygosity of these markers was retained at proximal, medial and distal regions of the extra chromosomes. These are the expected outcomes for non disjunctions in both M I and M II.