The C8orf13–BLK rs13277113A allele was associated with overall polymyositis/dermatomyositis (P<0.001, odds ratio [OR] 1.44, 95% confidence interval [CI] 1.19–1.73), as well as polymyositis (P = 0.011, OR 1.32, 95% CI 1.06–1.64) and dermatomyositis (P<0.001, OR 1.64, 95% CI 1.26–2.12). Here, BLK is linked to polymyositis.