C9orf72 and amyotrophic lateral sclerosis: Majounie et al. (2011) showed that the pathogenic GGGGCC hexanucleotide expansion in the C9orf72 gene associated with a high proportion of cases of amyotrophic lateral sclerosis and frontotemporal dementia was non-penetrant in individuals younger than 35 years, 50 % penetrant by age 58 but almost fully penetrant by age 80.