Chromosome 15q partial deletion is an extremely rare human genetic disorder, caused by a chromosomal aberration in which the long ("q") arm of one copy of chromosome 15 is deleted, or partially deleted. If the mother's copy of the chromosomal arm is deleted, Angelman syndrome results. If the father's copy of the chromosomal arm is deleted, Prader-Willi syndrome results. The smallest observed region that can result in these syndromes when deleted is therefore called the PWS/AS critical region and is situated at 15q11-q13. In addition to deletions, uniparental disomy of chromosome 15 gives rise to the same genetic disorders, indicating that genomic imprinting must occur in this region.