In MCF-7 human breast cancer cells, stimulation with epidermal growth factor (EGF), a ligand for the epidermal growth factor receptor (EGFR), or heregulin (HRG), a ligand for ErbB3/ErbB4 receptors, induces transient or sustained activity of intracellular kinases, depending on the ligand concentrations [2]. Here, EGF is linked to breast carcinoma.