Expression of uPA but not Plg by Lewis lung carcinoma. (A) Darkfield section of a subcutaneous tumor nodule that was hybridized with an antisense probe for uPA. Many of the tumor cells show a strong signal (bright white grains), indicating uPA mRNA expression. (B) Brightfield image of a higher magnification of the tumor cells shown in (A). Although there is cell to cell variability in the expression level of uPA, as shown in (A), the expression of uPA is localized to many of the tumor cells (arrows) forming the tumor nodule. Original magnifications: (A) ×100 and (B) ×650. (C) Northern blot of total RNA from subcutaneous tumors of control mice (lanes 2 and 3) and Plg^−/− mice (lanes 5 and 6) inoculated with Lewis lung carcinoma. Lanes 1 and 4 are total liver RNA purified in parallel from a control mouse and a Plg^−/− mouse, respectively. The positions of Plg mRNA, 28 S, and 18 S RNA are indicated. (D) Ethidium bromide staining of the agarose gel before blotting.