At the cellular level, estrogen binds to ERα leading to transcriptional activation of estrogen-responsive genes, which act on the signaling system for cell division and differentiation, among these genes are proto-oncogenes (such as c-fos, c-myc, and HER2/neu), cell cycle regulation cyclins, growth factors, etc. Membrane-bound G-protein-coupled estrogen receptor (GPER) binds to activate the second messenger system and promote tumor growth. The gene discussed is ERBB2; the disease is neoplasm.