CTSO and atrial fibrillation: Genetically elevated cathepsin O levels were significantly associated with increased AF risk (IVW: p = 0.0025, OR = 1.06, 95% CI 1.02–1.10), and this association remained robust in multivariable MR (IVW: p = 0.0265, OR = 1.0571, 95% CI 1.0065–1.1102).