<qa>  What is the in vitro binding of binders?4. Subjects must be binders to 11CPBR28 as determined via one of two ways: by prior participation in a PET scan with 11CPBR28 or by in vitro testing of their leukocytes. We found that non-binders have markedly reduced affinity of 3HPBR28 to leukocyte membranes. The in vitro binding has shown no overlapping values between binders 90 and non-binders. 