Genotype Cannot Be Raised by a Tumour

Resting heart rate predicts cancer mortality observationally, and the meta-analysis authors say plainly they cannot tell whether it is a marker of subclinical disease or a cause. Mendelian randomisation can: genotype is fixed at conception, so reverse causation is unavailable by construction. The instrument existed and nobody had pointed it at cancer. Run here on 278 variants against 76,192 breast cancer cases, it gives OR 1.115 per 10 bpm — landing almost on the observational estimate. A first pass with a named shortcut, not a settled result.