C9orf72 and frontotemporal dementia: The CRISPR/Cas9 system has been successfully used to establish C9orf72-deficient mice [56] since hexanucleotide repeat expansion in the C9orf72 gene has been recently discovered as a major cause of frontotemporal lobar degeneration (FTLD) with amyotrophic lateral sclerosis (ALS) [57].