CYP46A1 and Alzheimer disease: What is the reason for ectopic CYP46A1 expression in brain astrocytes in AD (Bogdanovic et al., 2001; Brown et al., 2004), in microglia in a mouse model of traumatic brain injury (Cartagena et al., 2008), and in macrophages in a mouse model of MS (Teunissen et al., 2007)?