C9orf72 and amyotrophic lateral sclerosis: Since CRISPR/Cas9-mediated genome editing may result in the risk of creating indels (Selvaraj et al., 2018), the combination of CRISPR/Cas9 genome editing and homology-directed repair (HDR) completely repaired the C9orf72 G4C2 repeat expansions to the wild-type repeat size in iPSC cells derived from ALS/FTD patient, and finally abolished pathological phenotypes (Ababneh et al., 2020).