CD31 staining demonstrates that ECs in human HCC xenografts within the mouse liver are of mouse rather than human origin. A) Representative immunohistochemical staining with a polyclonal antibody recognizing mouse and human CD31 demonstrates endothelial cells in sinusoid-like structures (arrows) within patient-derived human HCC xenografts in the murine liver (x200). B) Immunohistochemical staining of the same section as in Panel A with a monoclonal antibody against human CD31 is negative (x200), suggesting that ECs in the xenografts are of mouse origin. Similar staining patterns with the two antibodies are observed in intrahepatic xenografts generated from Huh7 cells (Panel C – polyclonal anti-mouse/human CD31, and Panel D – monoclonal anti-human CD31) and HepG2 cells (Panel E – polyclonal anti-mouse/human CD31, and Panel F – monoclonal anti-human CD31) (all x100). G) Positive control staining of murine kidney with polyclonal anti-CD31 demonstrates expected pattern of positivity of the glomerular endothelial cells (x200). H) Positive control staining of human kidney with monoclonal antibody against human CD31 demonstrates expected pattern of positivity of glomerular ECs verifying activity of the antibody used in Panels B, D, and F (x200). Staining of murine kidney with the monoclonal antibody against human CD31 was negative (not shown), confirming that this antibody is specific for human CD31.