The major cause of homocystinuria is mutation of the gene encoding the enzyme cystathionine beta-synthase (CBS). Deficiency of CBS activity results in elevated levels of homocysteine as well as methionine in plasma and urine and decreased levels of cystathionine and cysteine. Ninety-two different disease-associated mutations have been identified in the CBS gene in 310 examined homocystinuric alleles in more than a dozen laboratories around the world. Most of these mutations are missense, and the vast majority of these are private mutations. The two most frequently encountered of these mutations are the pyridoxine-responsive I278T and the pyridoxine-nonresponsive G307S. Mutations due to deaminations of methylcytosines represent 53% of all point substitutions in the coding region of the CBS gene.