['The G allele of NOTCH4 rs422951 is protective against demyelinating disease in Japanese.', 'The purpose of this study was to assess the relation of the G allele to neuromyelitis optica (NMO)/NMO spectrum disorder (NMOSD) and multiple sclerosis (MS) and the interaction between the G allele and HLA-DRB1 alleles, and to clarify any association of the G allele with clinical features.', 'DNA sequencing was used to genotype 106 NMO/NMOSD patients, 118 MS patients and 152 healthy controls (HCs) for rs422951.', 'G allele frequency in MS patients, but not that in NMO/NMOSD patients, was lower than that in HCs (8.9% vs 21.7%, p<0.0001, odds ratio (OR)=0.35). HLA-DRB1*0405 was positively associated with MS (OR=2.22, p(corr) =0.0380) while DRB1*0901 was negatively associated (OR=0.32, p(corr) =0.0114). Logistic regression analyses revealed that, after adjusting for gender and either HLA-DRB1*0405 or DRB1*0901, rs422951 was associated with MS in the dominant model (OR=0.37, 95% confidence interval (CI)= 0.20-0.66, p=0.0012). Haplotype analyses identified two susceptible and three resistant haplotypes formed from rs422951 and either HLA-DRB1*0405 or DRB1*0901. There were no statistically significant differences in clinical features between G allele carriers and non-G allele carriers.'] Does a NOTCH4 missense mutation confer resistance to multiple sclerosis in Japanese?