Since the first report of UPD in 1987, ∼3,300 cases of UPD have been described in the scientific literature8 (see Web Resources). To date, UPD of each of the autosomes and the X chromosome has been documented. UPD can cause clinical consequences by disrupting genomic imprinting or by unmasking harmful recessive alleles in large blocks of homozygosity on the affected chromosome. Detecting UPD is a useful diagnostic tool for specific imprinting disorders and for rare Mendelian diseases caused by homozygosity. UPD has also been implicated in tumorigenesis, particularly in cases of genome-wide UPD, which affects all the chromosomes in the genome. Thus, current understanding of UPD is based largely on case reports of individuals in which UPD is detected following suspicion of an imprinting or other clinical disorder, and, in some instances (typically