This theory has received a boost from a group of Dutch researchers led by Jan de Boer of Erasmus University in Rotterdam. De Boer's team began by creating mutant mice that aged prematurely because of accumulated damage to their DNA. In particular, the mice had a defect in an enzyme called a helicase that unwinds DNA so cellular machinery can fix it. At birth, the mice looked just like their normal littermates. But after about 3 months, the mutants' fur began to turn gray, and they became shrunken and, in many cases, infertile. By 14 months (normal mice live an average of 2 years), the surviving mutants had osteoporosis and their faces became tiny and bird-like.