Principles of MR and the assumptions that need to be satisfied to derive unbiased causal effect estimates. Dashed lines represent direct causal and potential pleiotropic effects that would violate MR assumptions. A indicates genetic variants used as IVs are only associated with the modifiable risk factor; B indicates genetic variants only influence the risk of developing MM through the modifiable risk factor; C indicates genetic variants are not associated with any measured or unmeasured confounders. SNP, single-nucleotide polymorphism.