"Soon after Hayflick discovered that cells are mortal, he realised that if you siphon some off each time they divide and freeze them, a single source can theoretically provide an almost unlimited supply – around 10,000,000,000,000,000,000,000 (10 sextillion) in total. And though WI-38 cells are mortal, because the cells had divided relatively few times when they were collected, they can be grown for longer before they reach the Hayflick limit. Most WI-38 cells have 50 divisions left, which each take 24 hours to complete, so they can be grown continuously for 50 days before you need to start again. Though there are hundreds of cell lines available in the United States, WI-38 makes up the majority of the cells used."