Looking at some of the genes already reported as potentially playing a role in AD [e.g., CDK5 (Liu et al., 2016)] or PD [e.g., COMT (Jiménez-Jiménez et al., 2014)], we observed that alterations in their expression were mostly driven by neuronal loss, i.e., by changes in cellular composition, but not so much by an intrinsic cell type-independent disease effect. The gene discussed is COMT; the disease is Alzheimer disease.