Interestingly, isolated (i.e. without methylmalonic acidemia) intermediate or even severe hyperhomocysteinemia—as observed in methylenetetrahydrofolate reductase (MTHFR) deficiency (OMIM 607093) or in homocystinuria due to cystathionine beta-synthase (CBS) deficiency (OMIM 236200)—have not been reported to cause any specific renal (e.g., HUS) or cardiopulmonary (e.g., pulmonary hypertension) disease apart from thromboembolism. Here, MTHFR is linked to methylmalonic acidemia.