HER receptor phosphorylation patterns. Phosphorylation levels of (A) EGFR, (B) HER2, and (C) 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+) during one hour time course after activation with low doses of EGF (12 ng/ml), HRG (40 ng/ml), or both (EGF+HRG). Samples were collected before ligand addition (t = 0), and at t = 10, 30, and 60 min after ligand addition. Receptor phosphorylation levels at these time points are presented as white, black, gray and cross line bars, respectively. Results shown here are the means and standard deviations (SD) of at least four 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.