In addition, Deo et al. (Deo et al., 2014) showed that in AD patients it was difficult to detect reduced P-gp function with (R)-[11C]verapamil, which was only successful when correcting K1 values for cerebral blood flow, illustrating the difficulties when using a substrate tracer for measuring expression of P-gp. This evidence concerns the gene PGP and Alzheimer disease.