Inhibition of HER receptor phosphorylation by Herceptin. HER2 blocking antibody Herceptin/4D5 was used in these experiments to examine the role of HER2 dimerization in the phosphorylation of HER3. Cells were preincubated with antibody Herceptin/4D5 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.