MTHFR and pulmonary arterial hypertension: 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.