To determine if the signaling responses that we observed in A431 cells were cell line-specific, we examined concentration-dependent signaling outcomes in three additional cell lines: MDA-MB-468 breast cancer cells, which, like A431, over-express EGFR (∼106 receptors per cell) [18], [19]; normal human mammary epithelial cells (HMECs; ∼105 receptors per cell) [20]; and human embryonic kidney cells that were stably transfected with EGFR (HEK293-EGFR; ∼105 receptors per cell) [17]. Here, EGFR is linked to breast cancer.