B7 expression by C1498 leukemia. C1498 leukemia cells or lines with either murine B7-1 or B7-2 cDNA introduction were stained with CTLA-4Ig and goat antihuman IgG biotin plus SA-PE and fluorescence intensity measured by flow cytometry. Control staining (lefthand figures) was performed in the absence of CTLA-4Ig. (A) C1498, (B) C1498B7-1, and (C) C1498B7-2.