Like a car's front and back bumpers, your cell's chromosomes are capped by "telomeres" that protect this genetic material against deterioration. Still, after enough replications, a chromosome's telomeres break down and once they reach a certain point of degradation, the cell dies. This is one reason that cells are mortal: telomeres only last so long. That is, unless the enzyme telomerase builds new material onto the worn telomeres to reinforce these chromosomal "bumpers". Telomere repair can be a good thing, but in some cases it's not: overactive telomerase can lengthen telomeres until a cell becomes immortal…leading to cancer.