The elevated level of Pgp in the perimeter of tumors (100 mm3) developed from hypoxic cells relative to the core may be due to the increased expression of HIF, as Pgp is transcriptionally activated by HIF, or may be a spatial response to microenvironmental factors as it is these cells in the perimeter of a tumor that are most likely to have the first contact with therapeutic agents. The gene discussed is PGP; the disease is neoplasm.