Experimental Schema. The primary aim of the current study was the development of an orthotopic model of multidrug resistant (MDR) breast cancer. To achieve that aim we conducted a study with two phases, in vitro and in vivo, as portrayed by the top and bottom portions of the figure. The first phase of the study consisted of exposing a panel of human cancer cells to either normoxic or hypoxic conditions and measuring the expression of protein markers for MDR, hypoxia, and glycolysis. These markers are portrayed by the cell diagram in the top, middle segment of the figure. The second phase of the study entailed selecting one cell line, exposing the human breast cancer cells to either normoxic or hypoxic conditions for five days, and then xenografting these cells into the mammary fat pad of nude mice. After tumors grew to 100 mm3, 250 mm3, and 500 mm3, they were excised and immunohistochemistry (IHC) was used to assess the expression of MDR, hypoxic, and glycolytic markers. Hypoxic pre-conditioning of xenografted cells did result in tumors with more MDR character than tumors established from normoxic cells. EGFR, epidermal growth factor receptor; HIF, hypoxia inducible factor; HXK2, hexokinase 2; Pgp, P-glycoprotein; GLUT1, glucose transporter 1.