Stromal elements in patient derived xenografts at termination of the first passage in SCID mice did not label with an anti-human markers; comparison of human markers in patient tumor specimens (A, B, C, G, H, I) and the first passage of the same tumor established as a xenograft (D, E, F, J, K, L). The anti-human mitochondrial antibody labeled tumor (single arrow) and stromal cells (double arrows) in patient specimens (A, B, C, G, H), but only tumor cells (single arrow) in the xenografts (D, E, F, J, K) while the stroma cells (double arrows) were unstained. Similarly, an anti-human CD34 antibody labeled vessels (arrow) in a patient tumor (I) but not in the xenograft (L). (A,D- Colon tumor; B, E- Lung tumor; C, F- Pancreatic tumor; G, J- RCC; H, K- Head & Neck tumor; I, L- Pancreatic tumor). (bars = 50 μ; except I, L bars = 100 μ).