EGFR and neoplasm: The finding of a relatively low incidence of detectable EGFR-GFP endocytosis in HSC3 tumor xenografts (Figures 2 and 3A) suggests that either the concentration of ligands accessible to EGFR in these tumors is quite low, resulting in activation of only a small pool of EGFRs, or EGFRs are incapable of efficient endocytosis in vivo.