Effect of cell density on HER receptor phosphorylation patterns. Phosphorylation levels of (A) EGFR and (B) HER3 are compared between parental cells (HER2-/3-), HER2 expressing cells (HER2+/3-), HER3 expressing cells (HER2-/3+), and HER2-HER3 co-expressing cells (HER2+/3+) after activation with low doses of EGF (12 ng/ml), HRG (40 ng/ml), or both (EGF+HRG) in cells grown to half or full confluency (described in the Methods section). The legend describes the time points and cell confluencies corresponding to each bar in the plot. Results shown here are the means and standard deviations (SD) of two measurements from two biological replicates. The data for each receptor have been normalized with respect to the receptor phosphorylation level of the HER2+/3+ cell line at t = 10 min when both ligands were added.