Scheme 1. Schematic representation of PMA signalling to Ras and the EGFR in COS-7 cells.

For reasons of clarity, two populations of EGFR are shown, one driving basal Ras-GTP formation, the second being targeted and transactivated in response to PMA (TPA). However, it is important to note that this study does not provide any evidence for the existence of different EGFR populations. The exact point of entry of ROS into the pathway is unknown.