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As previously reported, the C9orf72 mutation accounts for 23.5–47% of familial ALS/FTD and 4.1–21.0% of sporadic ALS in white populations ( Dejesus-Hernandez et al., 2011 Renton et al., 2011 Gijselinck et al., 2012). Additionally, the results of this study suggest the novel idea that the intermediate repeat allele in C9orf72 is most likely a risk factor for PD.Ī hexanucleotide (GGGGCC) repeat expansion in the first intron of the C9orf72 gene was recently identified as a major contributing factor to the chromosome 9p21-linked diseases amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) ( Dejesus-Hernandez et al., 2011). To the best of our knowledge, this study is the first to reveal the correlation between C9orf72 and Chinese PD, AD, or ET patients. However, the analysis of the association between the number of repeats ( p = 0.001), short/intermediate genotype (short: <7 repeats intermediate: ≥7 repeats) (odds ratio 1.37 ), intermediate/intermediate genotype (Odds ratio 2.03 ), and PD risks indicated that intermediate repeat alleles could act as contributors to PD. There were no pathogenic repeats (>30 repeats) detected in either the patients or controls ( n = 314), which indicated that the pathogenic expansions of C9orf72 might be rare in these three diseases. Using the repeat-primed polymerase chain reaction method, we screened for C9orf72 in three groups of patients with PD ( n = 911), AD ( n = 279), and ET ( n = 152) in the Chinese Han population. Given the overlapping of clinical phenotypes and pathological characteristics between these two diseases and Alzheimer's disease (AD), Parkinson's disease (PD), and essential tremor (ET), we speculated regarding whether C9orf72 repeat expansions also play a major role in these three diseases. GGGGCC repeat expansions in the C9orf72 gene have been identified as a major contributing factor in patients with amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD).