Interestingly, while complete loss of SLC1A3 function leads to a severe phenotype with progressive ataxia (Jen et al., 2005), mutation in transmembrane segment 4 has been associated with partial loss of function and variable penetrance (de Vries et al., 2009) which is consistent with a late onset disease such as ALS. The gene discussed is SLC1A3; the disease is amyotrophic lateral sclerosis.