Serial dilutions of HRG were incubated with 100 nM anti-EGFR and anti-ErbB3 mAbs alone, anti-EGFR and anti-ErbB3 mAbs in combination, the bsAb, or anti-EGFR/ErbB3 DVD-Ig proteins. HRG binding affinity to FaDu cells was measured via FACS. MFI: median fluorescence intensity. The error bars indicate standard deviation from the mean.