Genetic variation in the chromosome 9p21.2 locus, therefore, appears to be associated with altered gene expression of C9orf72. The recent discovery of the intronic hexanucleotide repeat expansion in C9orf72 on a common haplotype in 9p21.2 linked families with ALS and FTD [15], [16], [47] thus illustrates the potential of the combined use of gene expression and genotyping in search for causative genes in human diseases. Here, C9orf72 is linked to frontotemporal dementia.