Telomerase maintains and may even lengthen telomeres. Exposing human cells to telomerase slows cell aging and allows cells to begin copying again and longer telomeres cause gene expression to change to a younger phenotype which makes cells function as though they were younger. When telomeres get too short, our cells can no longer reproduce, which causes our tissues to degenerate and eventually die. Some cells, like those found in the skin, hair and immune system, are most affected by telomere shortening because they reproduce more often. Telomerase activation in aged or chronically stressed normal cells has been shown to slow or reverse telomere shortening, increase replicative capacity and restore or improve cellular function.