Molecular dynamics simulations of an atomic model of the transmembrane domain of the oncogenic ErbB2 receptor dimer embedded in an explicit dimyristoylphosphatidylcholine (DMPC) bilayer were performed for more than 4 ns. The oncogenic Glu mutation in the membrane spanning segment plays a major role in tyrosine kinase activity and receptor dimerization, and is thought to be partly responsible for the structure of the transmembrane domain of the active receptor. MD results show that the interactions between the two transmembrane helices are characteristic of a left-handed packing as previously demonstrated from in vacuo simulations. Moreover, MD results reveal the absence of persistent hydrogen bonds between the Glu side chains in a membrane environment, which raise the question of the ability for Glu alone to stabilize the TM domain of the ErbB2 receptor. Interestingly the formation of the alpha-pi motif in the two ErbB2 transmembrane helices confirms the concept of intrinsic sequence-induced conformational flexibility. From a careful analysis of our MD results, we suggest that the left-handed helix-helix packing could be the key to correctly orient the intracellular domain of the activated receptor dimer. The prediction of such interactions from computer simulations represents a new step towards the understanding of signaling mechanisms.