Roses: I reviewed Professor Smith's and Dr. Jobst's data with them at Oxford, and we are really not concluding anything very different. The tests of state used in their study, in this case the computed tomographic (CT) scan, work on essentially all patients with Alzheimer's disease (AD), no matter what their ApoE genotype. The positive predictor value of an E4 in the 60% or 75% of patients with AD who have a positive E4 is 97%, but for those 35% or so who are negative, the test has no value at all. So, in comparing its diagnostic usefulness, there will always be a proportion of patients for whom the test has no value.