Characteristics of participants
In the candidate targeted gene methylation detection stage, of the enrolled 100 patients and 100 HCs, one patient and one HC failed in bisulfite conversion assay. Detailed demographic and clinical characteristics were listed in Table 1. There were no significant difference in gender (χ2 = 1.172,P = 0.279) and mean age (t = 0.564, P = 0.574) existed between patients and HCs. In the second stage, 20 patients and 20 HCs were tested the mRNA level ofERAP1 gene. Similarly, no any significant difference in mean age (t = 0.102, P = 0.919) and gender (χ2 = 0.125,P = 0.723) were found. Data medication use and unilateral X-ray classification were only retrieved from a proportion of AS patients.