I've been curious about these so called immortal cell lines. Do they really just mean by immortal something qualitative and at the extremes, like some unreasonably high number of cell division cycles before senescence takes over and the tissue reaches end of line? I've been curious if anyone's ever sequenced the DNA from these lines to see if they're truly the same source organism or have diverged or another organism was slipped in their place unbeknownst to anyone not looking.