Expression of iNOS enzyme in transfected U937 cells. Western blot with a monoclonal antibody against mouse macrophage inducible NOS (iNOS) is shown. Lanes are numbered as follows: (1) M.W. markers; (2) untransfected U937 cells; (3) U937 cells transfected with the pCEP4 vector without the iNOS gene; (4) U937 cells transfected with the pCEP4 vector containing the iNOS gene; (5) lysate from murine macrophage RAW 264.7 cells induced with LPS and IFN-γ (positive control).