CD71 (transferrin receptor), a cell surface proliferation marker, is involved in the cellular uptake of iron (1-3). CD71 has been shown to exhibit strong membranous and cytoplasmic staining in all erythroid precursors of normal and dyspoietic bone marrow biopsies (1,2). CD71 expression decreases with the maturation of erythrocytes. The highest level is seen in early forms and the lowest level in late normoblast stage. Most importantly, mature erythrocytes do not express CD71, which facilitates bone marrow analyses (1,2). When compared to other biomarkers for erythroid precursers, such as hemoglobin or CD235a (glycophorin A), CD71 displayed the most specific staining with clean and distinct staining patterns and did not label mature red blood cells (2). CD71 was positive in all cases of parvovirus and acute erythroleukemia, unlike glycophorin A and hemoglobin A (1). CD71 did not stain benign lymphoid infiltrates or low grade lymphomas involving the marrow (1). CD71 may therefore be a reliable erythroid marker in bone marrow (1,2). Additionally, CD71 was shown to be highly expressed in invasive breast carcinoma with acquired resistance to tamoxifen (3). Abundant CD71 staining was also associated with poor prognosis in ER+/luminal-like breast cancer (3).