Based on the provided abstracts, there is insufficient information to characterize the function of the BTC (betacellulin) gene. The abstracts exclusively discuss biliary tract cancer (BTC) and its molecular characteristics, therapeutic targets, and treatment approaches 12345678. None of the provided literature addresses betacellulin as a growth factor or its role in EGFR signaling pathways. The abstracts focus on various aspects of biliary tract cancer including FGFR2 fusions, IDH1/2 mutations, HER2-targeted therapies, patient-derived organoids for drug screening, and precision medicine approaches. To properly summarize betacellulin's function as an EGF receptor family ligand and mitogen for retinal pigment epithelial and vascular smooth muscle cells, literature specifically addressing this gene's biological functions would be required.