MAPT and amyotrophic lateral sclerosis: In addition, although the A239T tau variant is reported to be benign (59,60), a recent study identified this mutation in a patient carrying a repeat expansion in the C9ORF72 gene (61), which is the most common cause of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD) (62,63).