The much debated difference in longevity between human genders might be due to differing levels of stem cell effectiveness. From EurekAlert!: "Female stem cells derived from muscle have a greater ability to regenerate skeletal muscle tissue than male cells ... Regardless of the sex of the host, the implantation of female stem cells led to significantly better skeletal muscle regeneration. Based on these results, future studies investigating regenerative medicine should consider the sex of the stem cells to be an important factor. Furthermore, investigations such as ours could lead to a better understanding of sex-related differences in aging and disease and could explain, at least partially, the high variability and conflicting results reported in the literature on stem cell biology. ... [researchers] injected female and male muscle-derived stem cells into dystrophic mice and [then] calculated the regeneration index (RI) ... Only one of the 10 male populations of implanted stem cells had an RI over 200. In contrast, 40 percent of the female stem cell populations had an RI higher than 200, and 60 percent of the female populations of stem cells had an RI higher than the mean RI of the male cells (95)."