CD9 is expressed on definitive erythroblasts in the FL and BM. CD9 expression was measured on Ter119⁺ cells from E14.5 FL (A, top panel) or adult BM (B, top panel) using analytical flow cytometry. Approximately 40% of Ter119⁺ cells from both tissues express CD9. (Middle panels) The developmental progression of erythroid cells from FL or adult BM was identified using CD71 and Ter119 surface expression. (Bottom panels) CD9 was present on most CD71⁺;Ter119⁻ (immature) erythroid cells (red), was down-regulated on CD71⁺;Ter119⁺ erythroblasts (blue), and was absent from mature CD71⁻;Ter119⁺ reticulocytes and erythrocytes (green) from both FL and adult BM.