How old is your four-legged best friend? Common wisdom says that a dog ages seven years for every human year. But Tina Wang, a graduate student at the University of California, San Diego, wanted a more accurate assessment. Her quest started five years ago, when she rescued a dog from a shelter. "l kind of just wanted to ask how old she was cause I felt like they were giving me an age that was very sensible, but I wasn't sure she was actually that old. So that kind of started this whole project." Wang worked in Trey Ideker's lab, where they study changing patterns of DNA methylation in humans. Small chemical entities, called methyl groups, attach to stretches of DNA, which affects what sequences are active. As we age, some stretches of DNA get more methylated, and others less. The pattern is so consistent over the course of most people's lives that it can be used as an aging "clock." The same process happens in dogs - and published reports existed from other labs about methylation patterns in dogs changing over time.