C9orf72 and Cognitive impairment: Although there was no association between genetic status and synapse density or cognitive impairment, it was interesting to note that 3 of our 4 genetically screened cognitively impaired cases were positive for C9orf72 repeat expansions (Fig. 2b), warranting further study of the effects of C9orf72 expansions, as more cases prepared for these techniques become available.