4.1. Cell Culture

A431 epidermoid carcinoma cells, Chinese hamster ovary (CHO) cells and transfected CHO cells stably expressing the fusion protein of the epidermal growth factor receptor and the F64L, S65T mutant of GFP (eGFP) [25] were grown in a 5% CO₂ humidified atmosphere at 37 °C in DMEM supplemented with 10% FCS. The subclone F1-10 used in this study expressed an estimated 2 × 10⁴ receptors per cell on average, as determined using the clone mAb 528 and Qifikit (DAKO / Agilent, Santa Clara, CA, USA) according to the manufacturer’s instructions. For EGF stimulation, 8 or 50 nM murine EGF (IC Chemikalien, Ismaning, Germany) was used. Prior to FCS stimulation, cells were kept in serum-free DMEM for 2 h to secure an EGF-free environment.