Issues that remain to be explored include the selectivity of P-site phosphorylation in heterodimeric receptors, such as in the highly transforming HER2/HER3 coreceptor combination that drives breast cancer progression [49], or how the binding of signaling effectors to phosphorylated P-sites as occurs in the cellular context would alter subsequent P-site phosphorylation. Here, ERBB2 is linked to breast cancer.