The discussion of Alzheimer’s disease, including discussion of the apolipoprotein gene variant APOE4, a widespread gene that increases susceptibility to Alzheimer’s disease, is illustrative. There are three rare single-locus mutations that independently cause Alzheimer’s disease in an infinitesimally small proportion of cases; and there is the APOE4 variant, which greatly increases risk and is epidemiologically much more common (found in one-fifth of Caucasians). APOE4 can be considered part of a multifactorial causative process for the common form of Alzheimer’s disease. In discussing the clinical applicability of APOE4 testing, the authors quote the American Society of Human Genetics, which does not recommend testing, and state that there is no compelling evidence supporting its routine use until more clinical research is completed. Now, the association between APOE4 and Alzheimer’s disease is arguably the most replicated genetic finding in inherited common diseases; the research that is needed is on whether the test results can be disclosed in a manner that can be useful to the patient. This is not clear from the authors’ discussion, however. They go on in the next section to a most informed discussion of how presymptomatic tests might be used in other psychiatric disorders with similar inheritance, but somehow the reader is not made aware of how close current knowledge of APOE4 is to their own criteria for a clinically applicable genetic test. Nonetheless, if this or another well-grounded finding comes under public consideration for use as a clinical predictive test, the reader of this book will have learned the principles needed to form his or her own judgment on whether the test should be used.