Dear Dr. Gott, Recently someone wrote in who was concerned about early onset Alzheimer's Disease (AD) due to the condition being prevalent in their family. They inquired about being tested for the genetic marker APO-E4 which is more common in AD patients. You suggested there was no value in this test and recommended the person not pursue this. As a chemist specializing in toxic metals analysis, I have to disagree with you as to the value of this test. High concentrations of apolioprotein E are found in the central nervous system. While the more common APO-E2 and APO-E3 contain the ammino acid cysteine, APO-E4 does not. Cysteine contains a sulfhydryl group which binds and flushes divalent metals, such as mercury, from the CNS. The role mercury plays is AD is a continuing controversial issue. If one tests positive for APO-E4, you may want to recommend that they steer clear of any unnecessary mercury exposure.