Myocardial infarction, a leading cause of death intheWesternworld(1), usually occurs when the fibrous cap overlying an atherosclerotic plaque in a coronary artery ruptures. The resulting exposure of blood to the atherosclerotic material then triggers thrombus formation, which occludes the artery(2). The importance of genetic predisposition to coronary artery disease and myocardial infarction is best documented by the predictive value of a positive family history(3). Nextgeneration sequencing in families with several affected individuals has revolutionized mutation identification(4). Here we report the segregation of two private, heterozygous mutations in two functionally relatedgenes, GUCY1A3 (p.Leu163Phefs*24) andCCT7 (p.Ser525Leu), in an extended myocardial infarction family. GUCY1A3 encodes the alpha 1 subunit of soluble guanylyl cyclase (alpha 1-sGC)(5), and CCT7 encodes CCT eta, a member of the tailless complex polypeptide 1 ring complex(6), which, among other functions, stabilizes soluble guanylyl cyclase. After stimulation with nitric oxide, soluble guanylyl cyclase generates cGMP, which induces vasodilation and inhibits platelet activation(7). Wedemonstratein vitro that mutations inbothGUCY1A3 and CCT7 severely reduce alpha 1-sGC as well as beta 1-sGC protein content, and impair soluble guanylyl cyclase activity. Moreover, platelets from digenic mutation carriers contained less soluble guanylyl cyclase protein and consequently displayed reduced nitric-oxideinduced cGMP formation. Mice deficient in alpha 1-sGC protein displayed accelerated thrombus formation in themicrocirculation after local trauma. Starting with a severely affected family, we have identified a link between impaired soluble-guanylyl-cyclase-dependent nitric oxide signalling and myocardial infarction risk, possibly through accelerated thrombus formation. Reversing this defect may provide a new therapeutic target for reducing the risk of myocardial infarction.