This figure shows the organization of the PWS locus on chromosome 15. Genes expressed from the paternal and maternal alleles are shown as white and grey squares, those expressed by both alleles are shown as ovals. SnoRNA genes are shown as vertical bars, classical break points 1, 2 and 3 are indicated as BP1,2 and 3. Imprinting center is shown as IC as an open circle when hypomethylated and black circle when hypermethylated . The critical minimum region for PWS is shown in the dotted frame.