The in vitro number of receptors per cell for each tumor cell line was determined as described previously (6 (link)). Briefly, when 80% confluence was reached, each cell line was labeled with 4 mg/mL of EGF Biotin (Molecular Probes, Invitrogen, Camarillo, CA, USA), and secondarily labeled with a 1:25 dilution of Cy5-Streptavidin (Invitrogen). Control cells were stained only with the Cy-5 Streptavidin to account for autofluorescence and non-specific staining. Quantification was performed by comparison to Quantum Cy5 MESF beads, used as described by the manufacturer (Bangs Laboratory, Fisher, IN, USA). Flow cytometric data was acquired with a FACSCalibur analysis system equipped with a FACStation, and Cell Quest Acquisition software (Becton Dickinson, San Jose, CA, USA). The average fluorescence measured for the control cells was subtracted from the EGF stained cells and the number of receptors per cell was calculated assuming one molecule of biotin per EGF and three Cy5 fluorophores per Streptavidin molelecule, as specified by Invitrogen.