Uniparental disomy (UPD), the inheritance of both homologues from one chromosome from the same parent, was first proposed in 1980 by Erik Engel  to be a potential cause of congenital developmental defects in hymans. First hints from the premolecular era towards its existence came from instances where a pericentric inversion was present on one homologue in a parent and on both in one offspring  and where there was transmission of an interhomologous Robertsonian translocation (of chromosome 22) from a healthy mother to healthy offspring [3-4]. In mice, UPD was experimentally produced by crossing two mice lines with different Robertsonian translocations both involving the same chromosome [for 2 review see ref. 5].Through this approach, it was possible to define imprinted regions, chromosomes and chromosomal segments for which either maternal or paternal or both types of uniparental disomy led to phenotypic abnormalities. The latter are explained by genomic imprinting, the differential silencing of a gene or genes from one of the parents (the mother or the father) during any stage of embryogenesis or later in life. If, for example, the maternal homologue of a given gene is imprinted (and hence only the paternal allele is active), maternal UPD would lead to loss of the active allele and thus might cause consequences due to loss of function.