Cystathionine beta synthase (CBS) is the only reaction that removes homocysteine from methionine cycle and redirects it to the transsulfuration pathway. The c.[833T>C;844_845ins68] mutation in the CBS gene has been reported initially as corresponding to classic homocystinuria. Studies showing that the insertion is associated with very smalls amounts of the transcript in the nucleus; others suggest that the heterozygous and homozygous subjects are protected against hyperhomocysteinemia and that the insertion tends to rescue the protein function. The liver is the major organ which metabolizes the circulating homocysteine to cystathionine. We have determined the sequence of the liver mRNA corresponding to the CBS c.[833T>C;844_845ins68] gene. We have shown that a novel splicing event could account for the modification in protein and possibly in enzyme activity.