Unlike P-GP, Xiong and colleagues reported that ABCG2 gene was significantly up-regulated in the brains of AD patients and AD animals (Xiong et al., 2009), however, this induction was not copied by in vitro study on hCMEC/D3 (Xiong et al., 2009; Kania et al., 2011). Here, PGP is linked to Alzheimer disease.