FGFR-1 expression by HCs. Receptor levels were measured by FACS using a triple-layer technique involving a mAb against FGFR-1, biotinylated antimouse antibody, and streptavidin-phycoerythrin (—). In all 6 cases, HCs were positively stained compared with the class-specific (IgG1) control (- - - -; representative result shown in panel A). Panel B shows the extent of positivity expressed as the mean fluorescence intensity (MFI) of the reactive cells.