CD44, cluster of differentiation; IDC-NOS, infiltrating ductal carcinoma-not otherwise specified; ILC, invasive lobular carcinoma; IPC, invasive papillary carcinoma; EPC, encapsulated papillary carcinoma; MUC C, mucinous carcinoma; ER, estrogen receptor; PR, progesterone receptor; HER2/neu, human epidermal growth factor receptor 2/neu. The gene discussed is ESR1; the disease is invasive lobular breast carcinoma.