Expression of MER2. (A) Flow cytometric studies of erythroid precursors (proerythroblasts) cultured from CD34⁺ cells. Dual staining with FITC-conjugated anti-GPA (anti-M) and PE-conjugated anti-MER2 (2F7) or anti-CD151 (11G5a) revealed a population of GPA⁺ erythroid cells that express CD151 (top right quadrant). (B) Changes in expression of MER2 (CD151) and other cell surface markers during ex vivo erythropoiesis. Continuous line indicates serologically MER2^– (without anti-MER2); dotted lines, MER2⁺. Days of culture are days after isolation of CD34⁺ cells. The dip in expression around day 7 is characteristic of these cultures. (C) Comparative levels of expression of MER2 (CD151) on erythrocytes and platelets of 5 donors. Tests with CD61 on platelets are included as a control. Samples 1 and 2 are MER2^– (without anti-MER2), and samples 3 to 5 are MER2⁺ as determined by an indirect antiglobulin test on erythrocytes. (D) Expression of MER2 (CD151) on lymphoblastoid cells. Flow cytometry histograms for lymphoblastoid cell lines derived from a MER2⁺ individual (gray peak) and from Pt2, the MER2^– patient with anti-MER2 (black peak), incubated with PE-conjugated anti-MER2, anti-CD151, and anti-CD37.