Human chromosomes a | 11p15.5 and b | 15q11–-q13 and orthologous clusters on a | mouse chromosome distal 7 and b | central 7. The relative location and transcriptional orientation of genes are indicated by arrows. The imprinting status is shown in red (maternally expressed), blue (paternally expressed), black (biallelic expression) and green (imprinted expression not known or not yet precisely defined). Question marks (?) indicate that the orthologues of the mouse or human genes, respectively, are not known. The drawings are not to scale. The Beckwith–Wiedemann (BWS) cluster (a) comprises about 1 Mb, and the Prader–Willi syndrome/Angelman syndrome (PWS/AS) cluster (b) roughly 2 Mb. Imprinting centres (IC) are marked by circles coloured according to the parental origin of the imprint.