Around mile 10 of a recent half marathon, my quadriceps started to tighten and my feet increasingly felt like lead. Along with improving my training, perhaps in the future I will use zinc-finger nuclease scissors to snip out a gene called IL-15Rα, so I can run long distances with ease.

Mice that lack this gene, which is related to muscle contraction, can run much farther than their counterparts, a new study says — suggesting a genetic predisposition to endurance in some athletes.

Physiologists led by Tejvir Khurana at the University of Pennsylvania were studying IL-15Rα, which had been linked to proteins associated with muscle contraction. They engineered mice to lack that gene, and recorded the mice’s activity. Every night, the knockout mice ran six times farther than normal mice, according to the Science NOW blog.

The team dissected the muscles in these marathon mice, and found the muscles had more fibers and more mitochondria, the power plants of cells, Science NOW reports. This meant the muscles took longer to tire and longer to use up their energy supplies. The researchers found that the lack of IL-15Rα coaxed one type of muscle cell to turn into another type — from fast-twitch, easily tired muscles into slower-contracting, longer-endurance muscles.