Discussion
Conceptualizing cancer in an evolutionary context promises to transform our understanding of the condition and offers new therapeutic possibilities [43]. The four members of the ErbB protein family are capable of forming homodimers, heterodimers, and possibly higher-order oligomers upon activation by a subset of potential growth factor ligands. However, their biological roles in many species are not clear. As the accumulation of gene sequences in the database occurs, it is feasible to explore evolutionary relationships and the functional diversity of the ErbB family. In this study, 62 sequences were used for phylogenetic reconstruction with Bayesian inference. Phylogenetic analyses showed the ErbB family formation of the paralogous lineages occurred before the divergence of individual species. We inferred that two major duplications had occurred early in the vertebrate lineages. The first duplication led to the emergence of two lineages, which evolved into EGFR and ErbB2, and the second duplication, also early in vertebrate evolution, resulted in ErbB3 and ErbB4. This result was similar with the functional divergence analysis. The results of phylogenetic analyses clearly constructed ErbB classification, which may be related to their functional divergence. Two main ligand classes have to date been identified for ErbB family. Signaling diversity depends both on the presence of specific receptors and the characteristics of individual ligands. Epidermal growth factor receptors differ in their ligand specificities. EGFR and HER2 classically couple to Ras-Raf-MEK-mitogen-activated protein kinase (MAPK)-dependent pathway, whereas HER3 is a potent activator of phosphatidylinositol 3-kinase (PI3-K)-Akt [44].