There was high fluorescence heterogeneity, within and around the tumors (Fig. 2a). Histograms of the tumor fluorescence, shown in Fig. 2b, demonstrate the regular presence of two regions of intensity. The regions of high ABY-029 fluorescence intensity were often well correlated to the regions of high EGFR expression, as illustrated by ex vivo staining using anti-EGFR antibodies in Fig. 2d. The EGFR-specific staining confirms the localization patterns expected; however, it is important to note that low level ABY-029 fluorescence is still visible in the regions with lower EGFR-specific staining with antibodies.Fig. 2. a The distribution of ABY-029 fluorescence in the tumor region immediately after tissue removal. b The heterogeneity of fluorescence signal distribution for each corresponding slice. c The H&E-stained tissue. d The EGFR-stained tissue. This information illustrates the true EGFR expression patterns.