Well, the observation that the two or three cultures were not behaving properly, suddenly stopped dividing after ten months or so, was no great shock, but when I looked at my records and discovered that they were the oldest ones, my ears perked up because that seemed to be inconsistent with what I had been taught and what I believed. As time went by, this phenomenon became more obvious as the cultures that were still dividing happily at that point themselves began to then stop dividing. So that it became perfectly obvious that the oldest cultures were the ones that... in which division capacity had stopped. And that was... that seemed to be a true case, but needed to be proven because again, at that time, one of the other reasons invoked for failures of cultures to continue to divide was the appearance of viruses. Virology, as I said several times I think, was now in its renaissance and many virologists were working with cell cultures in which research could more easily be done than in animals and viruses were always on my mind because most of the people around me at the institute were virologists, or working in some way with them. So that was a serious consideration, what’s causing the cessation of mitonic activity in these cells.