TGFA and neoplasm: For example, the ErbB receptor family, one of the most studied receptor tyrosine kinases in vivo and in vitro, is activated by various types of ligands including epidermal growth factor (EGF), transforming growth factor alpha (TGF-α), and heregulin (HRG), leading to widespread phosphorylation of representative downstream signaling cascades such as mitogen-activated protein kinase (MAPK) and phosphoinositide 3-kinase (PI3K)/AKT signaling pathways [14]–[16] to promote various types of tumor development.