MTHFR and Patent foramen ovale: The relationship found between the L-Arg/ADMA ratio and the PFO tunnel morphology might partially explain why PFO prevalence decreases with age, whereas its size increases [49]; this idea is consistent with the observation that reduced MTHFR activity contributes to impair survival and function of circulating endothelial progenitor cells [50], whose inefficiency is important on stroke onset [51].