Besides general quality control measures, additional factors must be considered when designing experiments to profile human postmortem brain. First, alcoholic individuals with a history of polydrug abuse should be excluded or appropriately controlled. Polydrug use can be considered an exclusion factor but this may depend on the population cohort. Although challenging, one could control for polydrug use by using appropriate control groups or by using statistical covariates. It should be noted that there is a high incidence of smoking among alcoholics and nicotine use represents an important covariate to investigate. Gene expression differences between control subjects and alcoholics can thus potentially be driven by nicotine dependence or the interaction between nicotine and alcohol. Second, cases with concomitant diseases such as cirrhosis of the liver, Wernicke-Korsakoff syndrome, or hepatic encephalopathies should also be considered in the experimental design. As with polydrug use, exclusion of concomitant diseases is not necessarily a requirement for all studies. Concomitant diseases as an experimental factor in study designs can be useful in creating more homogenous groups and reducing heterogeneity (Liu et al., 2004; Liu et al.,
