The study investigates the expression of P-glycoprotein (Pgp), a multidrug-resistance gene product, in endothelial cells at blood-brain and other blood-tissue barrier sites. Using mouse monoclonal antibodies, the researchers found that Pgp is highly expressed in the endothelial cells of capillary blood vessels in the central nervous system, testes, and papillary dermis, but not in other tissues such as the kidney and placenta. This expression pattern suggests a physiological role for Pgp in regulating the entry of certain molecules into these specific anatomic compartments, which may explain why systemic chemotherapy often fails to eradicate cancer cells in these "pharmacologic sanctuaries" like the brain and testes. The findings have implications for developing therapeutic strategies to overcome drug resistance in cancer cells while minimizing host toxicity.The study investigates the expression of P-glycoprotein (Pgp), a multidrug-resistance gene product, in endothelial cells at blood-brain and other blood-tissue barrier sites. Using mouse monoclonal antibodies, the researchers found that Pgp is highly expressed in the endothelial cells of capillary blood vessels in the central nervous system, testes, and papillary dermis, but not in other tissues such as the kidney and placenta. This expression pattern suggests a physiological role for Pgp in regulating the entry of certain molecules into these specific anatomic compartments, which may explain why systemic chemotherapy often fails to eradicate cancer cells in these "pharmacologic sanctuaries" like the brain and testes. The findings have implications for developing therapeutic strategies to overcome drug resistance in cancer cells while minimizing host toxicity.