CBS, Cystathionine Beta-Synthase , catalyzes the first irreversible step of transsulfuration.The CBS enzyme is a homotetramer of 63-kD subunits and requires pyridoxal phosphate and heme for activity. The CBS gene, which is mapped to chromosome 21q22 , contains 23 exons, ranging in size from 42 to 299 bp.The human CBS protein can substitute for the endogenous yeast CBS protein in Saccharomyces cerevisiae.The catalytic domain of the CBS protein is located in the N-terminal 409 amino acids, and a regulatory domain is located in the C-terminal 142 amino acids.A mutation that deletes the C-terminal 145 amino acids of CBS could restore activity of several CBS mutant alleles found in homocystinurics.