In turn, current smoking status was causally associated with IL-2-sRβ plasma levels (MR β = −0.18, 95% CI −0.26 to −0.10; tier 2 finding) (Supplementary Table 15), suggesting a possible biological mechanism for the effect of smoking cessation on reducing adult-onset asthma risk (causal OR 1.55, 95% CI 1.16 to 2.07). Here, IL2 is linked to adult onset asthma.