Unmixed cell populations were kept as controls. When the male control culture stopped dividing, the mixed culture was examined and only female cells were found. This showed that the old male cells remembered they were old, even when surrounded by young cells, and that technical errors or contaminating viruses were unlikely explanations as to why only the male cell component had died.[1][3] The cells had stopped dividing and had become senescent based purely upon how many times the cell had divided.