The association of the T-allele of MTHFR C677T (A222V) with hyperhomocysteinemia, cardiovascular disease and stroke suggests a causal relationship between hyperhomocysteinemia and vascular disease, and a meta-analysis stated evidence for causality from Mendelian randomisation, arguing that the observed increase in the risk of stroke among individuals homozygous for the T-allele is close to that predicted from the differences in homocysteine concentration conferred by this variant [1]. The gene discussed is MTHFR; the disease is hyperhomocysteinemia.