As you read this article, the cells in your body are dividing and the DNA in them is being copied, letter by letter. So long is the human genome—more than 3 billion letters—that even an astonishingly low error rate of one in many million letters could amount to 10 new mutations every time a cell divides.
https://www.theatlantic.com/science/archive/2018/05/your-body-acquires-trillions-of-new-mutations-every-day/559472/