 Under the current and other protocols using 11CPBR28, we found that some healthy subjects and patients have very low to no binding of 11CPBR28. We studied approximately 188 subjects in total under protocols using 11CPBR28 and found that 8.5 16188 had almost no binding of 11CPBR28. Several published ligands have been tested and all of the tested ligands recently developed show low affinity to a subset of humans. By using 11CPBR28, we need to exclude PBR28 nonbinders from the data to study changes in PBR. By using a PET ligand that binds equally to PBR28 binders and nonbinders, we would be able to study all subjects. We also confirmed that PBR28 nonbinders determined by PET do not show binding in in vitro binding assays using blood cells. 