A gene comprises a DNA sequence, including introns, exons and regulatory regions, related to transcription of a given RNA. They provide the template for production of a specific protein.
Some genetic variants that are used in MR are within the coding region of a known gene. When MR is used to test the effect of a product of a known gene (e.g., adiponectin or C-reactive protein) on an outcome, variants in the genes coding that protein are used as IVs. Bias due to horizontal pleiotropy is less likely than with variants whose function is unknown or where the risk factor being tested is complex and not a direct gene product (e.g., body mass index [BMI]).