A 53-year-old woman discovers a lump in her breast and physical examination confirms a mass in the lower, outer quadrant of the left breast. Mammography demonstrates an ill-defined, stellate density measuring 1 cm. Needle aspiration reveals malignant ductal epithelial cells. A modified radical mastectomy is performed. The surgical specimen reveals a firm irregular mass (arrows). Which of the following cellular markers would be the most useful to evaluate before considering therapeutic options for this patient?

Over half of breast cancers exhibit nuclear estrogen receptor protein. A slightly smaller proportion also has progesterone receptors. Women whose cancers possess hormone receptors have a longer disease-free survival and overall survival than those with early-stage cancers who are negative for these receptors. The beneficial effects of oophorectomy on survival in patients with breast cancer led to the use of estrogen antagonists in the treatment of breast cancer. In general, antiestrogen therapy seems to prolong disease-free survival, particularly in postmenopausal and node-positive women. It also lowers the risk of cancer in the contralateral breast. The latter discovery has led to the use of antiestrogens as chemoprevention in women at high risk for developing breast cancer. None of the other choices are related prognostically to breast carcinoma.Diagnosis: Invasive ductal carcinoma of the breast