Prader-Willi syndrome (PWS) and Angelman syndrome (AS) are caused by deficiencies of gene expression from paternal or maternal chromosome 15q11-q13, respectively. Many advances have occurred during the past year. The gene for necdin was mapped in the PWS candidate region and found to be paternally expressed in mouse and human. The bisulfite method for analysis of methylation was established for genomic sequencing and diagnostics, and the methylation of Snrpn was studied in detail in the mouse. A region near the Snrpn promoter was shown to function as a silencer in Drosophila. Point mutations were found in the gene for E6-AP ubiquitin-protein ligase (UBE3A) identifying it as the AS gene, and tissue-specific imprinting (maternal expression) was shown in the human brain and in hippocampal neurons and Purkinje cells in the mouse.