To determine BTB passive permeability and whether P-gp influences R123 uptake into brain metastases of breast cancer, tumor-bearing mice were injected with 14C-AIB (passive permeability tracer) which was allowed to circulate for 10 min before a 2 min R123 perfusion, which was followed by sacrifice (Figure 3). The gene discussed is PGP; the disease is breast carcinoma.