Epigen was initially identified in mice by high throughput sequencing of a mouse keratinocyte library. It belongs to the EGF family of proteins that includes EGF, TGF-α, heparin-binding EGF like-growth factor (HB-EGF), amphiregulin, epiregulin, and betacellulin. Epigen shows 24-37% identity to other members of the EGF superfamily. Alignment of the EGF-like motifs of all human ErbB ligands indicate that epigen is more similar to epiregulin than other members of the family. In addition, epigen and epiregulin co-localize to the same genomic locus. All the EGF family members are synthesized as type I membrane protein precursors, which are able to undergo proteolytic cleavage at the plasma membrane to release a mature soluble ectodomain. In addition to amphiregulin, TNF-α, and HB-EGF, ADAM 17 (TACE) has an important role in ectodomain shedding of epigen. This cleavage is a key step in the control of ligand availability and receptor activation and is stimulated by phorbol esters, phosphatase inhibitors, and calcium influx. The four cell surface receptors specific for EGF-ligands are members of the ErbB family of tyrosine kinase receptors. These include the epidermal growth factor (EGF) receptor (EGFR/ErbB1), ErbB2/HER2/Neu, ErbB3/HER3, and ErbB4/HER4. Epigen induces proliferation of cells engineered to express EGFR, either alone or in combination with ErbB2. HER2 does not bind to any ligand, but it can form relatively potent receptor heterodimers. Epigen is expressed in epithelial cells of invasive breast, prostate adenocarcinomas, and in head and neck squamous cell carcinoma (HNSCC) cell lines. It is overexpressed in bladder and breast cancer cells.