Published and our own data, included in the CHRODYS database, on the dependence of phenotypic abnormalities in mono-, di-, and trisomics at human chromosome 15 on its parental origin are reviewed. The concept is confirmed that Prader-Willi and Angelman syndromes result from the combined effect of gene or chromosome mutations impairing the expression of syndrome-specific genes and from genomic imprinting, i.e., repression of corresponding genes received from one of the parents.