We found that co-transduction of C9-ALS/FTD iMNs resulted in a shift from amplification of a 523 bp product to the 320 bp product in cells treated with AAV9–SpCas9 and either gRNA pair, but not in C9-ALS/FTD or fragile X syndrome control iMNs without transduction with AAV9–SpCas9 (Fig. 5a). Here, C9 is linked to fragile X syndrome.