Inhibition of HER receptor phosphorylation by 2C4. HER2 blocking antibody 2C4 was used in these experiments to examine the role of HER2 dimerization in the phosphorylation of (A) EGFR, (B) HER2, and (C) HER3. Cells were preincubated with antibody 2C4 for 4 hours before addition of 12 ng/ml EGF, 40 ng/ml HRG, or both. The white, black and gray bars represent the receptor phosphorylation levels at t = 0, 10 and 30 min, respectively. The means and SD of two biological replicates are presented.