EGFR and HER3 phosphorylation in response to high ligand doses. Phosphorylation levels of (A) EGFR and (B) HER3 were measured in the HER3 expressing cell HER2-/3+ during a two hour time course following activation with 100 ng/ml EGF, 80 ng/ml HRG, or both ligands simultaneously. The white, black, gray, cross line and dotted bars represent the receptor phosphorylation levels at t = 0, 10, 30, 60 and 120 min, respectively. Results shown here are the means and SD of two biological replicates. The data in each panel was normalized with respect to the phosphorylation level of the respective receptor at t = 10 min when both ligands were added.