A supernumerary marker chromosome 15 tetrasomic for the Prader-Willi/Angelman syndrome critical region in a patient with a severe phenotype

The proximal region of chromosome 15q is predisposed to a wide range of structural rearrangements. Deletions of this region, spanning approximately 4 Mb, can be paternally or maternally derived and result in Prader-Willi syndrome and Angelman syndrome, respectively.1 Additional copies of the Prader-Willi/Angelman syndrome critical region (PWACR) can occur as interstitial duplications and triplications or as supernumerary marker chromosomes (SMCs).