A group of genes linked to Prader-Willi syndrome (PWS)-a rare disorder that includes cognitive disabilities and obesity-are reported in a paper published online this week in Nature Genetics. The study sheds light on the cause of PWS.

PWS occurs when a group of genes on the copy of chromosome 15 inherited from the father are silenced. Nissim Benvenisty and colleagues used stem cells created from PWS patient cells to find out why the silenced paternal genes were so important. They discovered that a small gene called IPW is necessary for the normal function of a group of genes on an entirely different chromosome. Interestingly, those genes are only active on the chromosome copy inherited from the mother.