Polymorphisms in specific genes have also been linked to MN vulnerability. In SALS patients, variants in the gene encoding UNC13A are associated with greater susceptibility to disease and shorter survival (Diekstra et al., 2012). UNC13A functions in vesicle maturation during exocytosis and it regulates the release of neurotransmitters, including glutamate. Mutations in EPHA4 are also associated with longer survival (Van Hoecke et al., 2012), implying that Epha4 modulates the vulnerability of MNs in ALS. Furthermore, repeat expansions in the gene encoding ataxin 2 (ATXN2), which cause spinocerebellar ataxia type 2 (SCA2), are also increased in ALS patients compared to healthy controls (Ross et al., 2011). This implies that ATXN2 repeat expansions are also related to MN vulnerability to neurodegeneration in ALS.