sentences:  However, finding conclusive evidence against the immortal strand hypothesis has proven difficult.After Cairns first proposed the immortal DNA strand mechanism, the theory has undergone several refinements.In 2002, he proposed that in addition to using immortal DNA strand mechanisms to segregate DNA, when the immortal DNA strands of adult stem cells undergo damage, they will choose to die (apoptose) rather than use DNA repair mechanisms that are normally used in non-stem cells.