FKBP4 and Alzheimer disease: FKBP with a molecular mass of ∼52 kDa (FKBP52) is one of the most well-studied FKBPs, and FKBP52 has been shown to be highly expressed in neurons and abnormally low in AD brains (Giustiniani et al., 2012, 2014, 2015).