Regional brain MAO-B activity is increased in AD (Adolfsson et al., 1980; Oreland and Gottfries, 1986) and mean platelet MAO-B activity is increased in female AD patients (Robinson et al., 1971; Veral et al., 1997), yet it is not clear how much of this change might rely on the patient’s biological sex or on other factors such as the widely-acknowledged genetic risk for late-onset AD, i.e., the APOE ε4 allele (Poirier et al., 1993). The gene discussed is MAOB; the disease is Alzheimer disease.