TTN and familial dilated cardiomyopathy: Similar, “hypothesis-led” designs underpinned candidate gene and linkage studies that established causal relationships between rare genetic variants and rare conditions, such as those that first identified the role of myosin heavy-chain beta in hypertrophic cardiomyopathy (HCM) (9) and of titin in dilated cardiomyopathy (DCM) (10).