To the Editor.—In commenting on the papers concerning cystathioninuria, which were published as part of the "Symposium on Treatment of Amino Acid Disorders" (Jan 1967), Dr. Gerald Gaull1 makes several factually incorrect comments concerning the work performed by our group.2 His statement that we do not add pyridoxal phosphate to the cystathionase assay system is specifically refuted by the published description of our method3 which was reiterated and discussed in our paper on cystathioninuria.2 Indeed, in our discussion we note that "our studies show that in spite of both in vitro and in vivo administration of vitamin B6 cystathionase activity in this patient could not be restored beyond a level far below normal."2
Gaull also remarks that our assay "depends for its sensitivity on just a few counts above background." This is not so. In the paper in which our method was described