To define the exact role of relatively common genetic variants in Mendelian disorders much more research is required. Broad genetic screening and genome-wide association studies (GWAS) do have relevance for scientific studies to establish whether variants are genetic modifiers, which may be either protective or detrimental. GWAS analysis in a group of patients with Brugada syndrome revealed a cumulative effect of three loci on disease susceptibility, indicating an important modulating role for genetic polymorphisms in cardiac conduction disorders [
3]. After the first identification of variants in the gene encoding the giant protein titin as cause of dilated cardiomyopathy [
4], follow-up studies indicated that these titin gene variants are less pathogenic when compared with, for instance, lamin variants [
5]. Because of its large size, many relatively common gene variants are found in titin. While titin gene variants appear to be less harmful compared with other dilated cardiomyopathy genes, studies in patients carrying titin gene variants showed that increased cardiac stress during an ‘acute’ event such as pregnancy initiates disease [
6]. Moreover, other cardiac stress inducers such as chemotherapy and alcohol predispose persons with gene variants to development of disease [
7,
8]. Even proven disease-causing variants show a large clinical heterogeneity in families, indicating that the actual onset and progression of cardiomyopathy is the result of the combined effect of gene variants and additional genetic and environmental modifiers. Knowledge of the role of genetic background (polymorphisms) in addition to the pathogenic variant and environmental factors is needed to fully understand the complex and heterogeneous clinical phenotypes, which can range from asymptomatic persons to persons with end-stage heart failure in one family sharing the same pathogenic variant.