C9orf72 and amyotrophic lateral sclerosis: By genotyping the repeat sizes of the non-expanded GGGGCC alleles in all the C9-ALS patient and WT control cell lines used in our study through Sanger sequencing, we confirmed that all the WT control cell lines, as well as the HeLa and HEK293 cells, carried 2× GGGGCC repeat, which is the most common form of non-expanded C9orf72 alleles (Supplemental Table 1).