Maytansine's structure enables it to stop a cell from dividing, thereby killing the cell.

WSJ: Drug Conjugates: 'Guided Missiles' to Treat Cancer

An aging stem cell doesn't quite have the luxury of dividing into all these cell lines.

CNN: Dr. Sanjay Gupta: Understanding stem cell research

Cell biologists led by Judith Campisi at the Lawrence Berkeley National Laboratory in California doubt that every cell has this dividing limit, and believe that it could be only those cells that have stopped dividing that cause ageing.

ECONOMIST: The latest from the wacky world of anti-senescence therapy

This revealed the four-stranded DNA arose most frequently during the so-called "s-phase" when a cell copies its DNA just prior to dividing.

BBC: 'Quadruple helix' DNA seen in human cells