Human cystathionine .beta.-synthase variants are disclosed, as well as a method to produce recombinant human cystathionine .beta.-synthase and variants thereof. More particularly, the role of both the N-terminal and C-terminal regions of human CBS has been studied, and a variety of truncation mutants and modified CBS homologues are described. In addition, a method to express and purify recombinant human cystathionine .beta.-synthase (CBS) and variants thereof which have only one or two additional amino acid residues at the N-terminus are described.