Abnormalities of chromosome region 15q11-13 are associated with Angelman syndrome (AS) and Prader-Willi syndrome (PWS). Differences between the methylation patterns of the region of chromosome 15q11-13 which hybridizes to the highly conserved DNA, DN34, in normal individuals and in patients with AS and PWS have been described. We report on a family in which first cousins are affected by AS and PWS as a result of a familial paracentric inversion of 15q11-q13. The results of the studies on this family demonstrate the differences in the methylation patterns in the 2 conditions and the phenomenon of genomic imprinting, whereby genetic information is expressed differently dependent on the parent of origin.