C9orf72 is found in many regions of the brain, in the cytoplasm of neurons as well as in presynaptic terminals.The mutation of C9ORF72 is a hexanucleotide repeat expansion of the six letter string of nucleotides GGGGCC. In a normal person, there are up to 30 repeats of this hexanucleotide, but in people with the mutation, the repeat can occur in the order of hundreds. It is known that the mutation interferes with normal expression of the protein made by C9orf72, however the function of that protein is unknown. There are two major theories about the way that the C9ORF72 mutation causes FTD and/or ALS. One theory is that accumulation of RNA in the nucleus and cytoplasm becomes toxic, and RNA binding protein sequestration occurs.