During the aging process, a decline in the normal cellular ability to maintain proteostasis is observed and, as a result, damaged proteins accumulate (Kikis et al., 2010). Thus the normal aging process in MNs that are already weakened by ALS-associated insults, such as the presence of misfolded proteins or environmental factors, may combine to induce neurodegeneration. MN populations that are more susceptible in ALS may therefore be less able to tolerate disturbances in proteostasis than the more resistant populations (Neumann et al., 2006; Kikis et al., 2010).