MtDNA mutations and the aging process: a summary of hypotheses. Whereas all of these hypotheses are plausible, it is not clear which pathway(s), if any, are important for normal aging. More research, in particular detailed measurements of mutant fractions in normal and mutator mice and in humans, is necessary to evaluate these diverse hypotheses. The hypotheses do not need to be mutually exclusive; in fact, it is quite possible that different types of mutations have different roles in various cell types.