Any amyotrophic lateral sclerosis in which the cause of the disease is a mutation in the HNRNPA1 gene. The disease is amyotrophic lateral sclerosis type 20 (MONDO_0014181). Also known as: ALS20, HNRNPA1 amyotrophic lateral sclerosis, amyotrophic lateral sclerosis 20, amyotrophic lateral sclerosis caused by mutation in HNRNPA1.