There is vitamin B6 – unresponsive and vitamin B6 – responsive cystathioninuria. The vitamin B6 – unresponsive form is thought to be from a lack of the synthesis of cystathionase. This means that the mutation in CTH from this form of cystathioninuria, results in the absence of cystathionase.
Two forms of cystathioninuria are distinguished: vitamin B6 – unresponsive and vitamin B6 – responsive. The unresponsive form is thought to arise from an inability to synthesize cystathionase, which leads to a deficiency in this enzyme due to a mutation in the CTH gene.