To evaluate C9orf72 iPSC-MG for C9orf72-specific disease mechanisms, we first tested C9orf72 iPSC-MGs for haploinsufficiency by examining the levels of the C9orf72 transcript in C9orf72 ALS/FTD and control iPSC-MG. Here, C9orf72 is linked to frontotemporal dementia.