In 1961, cellular aging was first described by Hayflick and Moorhead. They showed that human cells in culture do not divide indefinitely but reach a limit (called the Hayflick limit) of replication and stop all further division. Cells approach this limit by slowing their divisions and entering cellular senescence, a dormant period. Recently, for damaged cells, this pathway of cellular progression has been considered an alternative to apoptosis (cell suicide). Both DNA damage and insufficient telomere replication are common signals leading to these events. When the cell does not trigger either of these pathways, it can become cancerous.