Preprogrammed cell death. The genes responsible are also known as suicide genes. A natural mechanism for preventing cancer in higher organisms. Cells which are damaged, or old, or which have outlived their usefulness receive a molecular trigger that causes them to simply shrivel up and die without disrupting the cells around them. When apoptosis fails, cell lines can proliferate out of control, which can result in cancer. It should be noted that preventing these genes from acting will not grant immortality. Cells have a finite useful life, largely due to the fact that chromosomes lose some genetic material each time they replicate. Cells have to die when their chromosomes get too short, otherwise valuable genes would be lost each cycle of cell division. The ends of chromosomes are replenished using the enzyme Telomerase, which adds many repetitions of a short sequence to the end. Normal human cells have no Telomerase activity, except at a very low level in the gonads. Cancerous tumors do have telomerase activity.