The homocysteine plasma level is determined by non-genetic and genetic factors. In recent years evidence has accumulated that the total homocysteine plasma level of patients under different forms of renal replacement therapy is influenced by a common mutation at nucleotide position 677 of the gene coding for 5,10-methylenetetrahydrofolate reductase (MTHFR 677C-->T). Furthermore, compound heterozygosity for the 677T allele and a novel A-->C polymorphism at nucleotide position 1298 of MTHFR is suggested to correlate with a decrease of folate plasma concentrations. Because polymorphisms of genes coding for proteins involved in the metabolism of homocysteine may contribute to elevated total homocysteine plasma concentrations, molecular genetic analyses of the homocysteine pathways experienced a drift towards screening for candidate genes with a putative relationship to total homocysteine plasma levels. One example is the cloning of the FOLR1 gene coding for the folate-binding protein (Folbp1), which has recently been inactivated in mice, thus representing an elegant model to investigate the consequence on the homocysteine metabolism. Furthermore, the recent characterization of the CUBN gene encoding the intrinsic factor-vitamin B12 receptor (cubilin) provides a basis to identify the causative mutations in patients suffering from a hereditary syndrome of hyperhomocysteinemia that presents with megaloblastic anemia and proteinuria. This review focuses on recent insights into the molecular genetics of MTHFR, FOLR1, and CUBN, and their relationships to the metabolism of the amino acid homocysteine.