Aim of the study: Thiopurines are effective drugs in treating neuromyelitis optica spectrum disorders and other diseases. Thiopurines' toxicity is mainly imputed to thiopurine S-methyltransferase activity. In Chinese population, the most common and important variation of thiopurine S-methyltransferase is TPMT*3C (rs1142345). This study aims to reveal the association between thiopurine S-methyltransferase activity and genetic polymorphisms of thiopurine S-methyltransferase in patients with neuromyelitis optica spectrum disorders in China.Material and methods: A liquid chromatography tandem mass/mass method was used to evaluate the thiopurine S-methyltransferase activity by using 6-mercapthioprine as the substrate in human erythrocyte haemolysate via 1h incubation at 37 degrees C to form its methylated product 6-methylmercaptopurine. The amount of 6-methylmercaptopurine was adjusted by haematocrit and normalized to 8x 10(8) erythrocytes. The selected polymorphisms of thiopurine S-methyltransferase were identified using MassARRAY system (Sequenom) and multiple SNaPshot technique.Results: In 69 patients with neuromyelitis optica spectrum disorders, thiopurine S-methyltransferase activity was 80.29-154.53 (127.51 16.83) pmol/h/8x 10(8) erythrocytes. TPMT*3C (rs1142345) was associated with lower thiopurine S-methyltransferase activity (BETA = -25.37, P = 0.011). Other selected variants were not associated with thiopurine S-methyltransferase activity.Conclusions: TPMT*3C affects TPMT activity in Chinese patients with neuromyelitis optica spectrum disorders. Further studies are warranted to confirm the results.Abbreviations:TPRs = thiopurines; NMOSD = neuromyelitis optica spectrum disorders; TPMT = thiopurine S-methyltransferase; LC-MS/MS = liquid chromatography tandem mass/mass; 6-MMP = 6-methylmercaptopurine; IS = internal standard; SNP = single nucleotide polymorphism; MAF = minor allele frequency; HWE = Hardy-Weinberg equilibrium; BETA = regression coefficients; UTR-3 = untranslated region 3