Severely elevated plasma homocysteine(Hcy) levels observed in genetic disorders of Hcymetabolism are associated with pathologies in multipleorgans and lead to premature death due to vascularcomplications. In addition to elevating plasma Hcy,mutations in cystathionine ␤-synthase (CBS) or methyl-enetetrahydrofolate reductase (MTHFR) gene lead tomarkedly elevated levels of circulating Hcy-thiolactone.The thiooester chemistry of Hcy-thiolactone underliesits ability to form isopeptide bonds with protein lysineresidues (N-Hcy-protein), which may impair or alter theprotein’s function. However, it was not known whethergenetic deﬁciencies in Hcy metabolism affect N-Hcy-protein levels in humans. Here we show that plasmaN-Hcy-protein levels are signiﬁcantly elevated in CBS-and MTHFR-deﬁcient patients. We also show that CBS-deﬁcient patients have signiﬁcantly elevated plasma levelsof prothrombotic N-Hcy-ﬁbrinogen. These results pro-vide a possible explanation for increased atherothrombo-sis observed in CBS-deﬁcient patients.