The 2B7 antibody reacts with CD235a (Glycophorin A), an ~10 kDa type I sialoglycoprotein present in the cell membrane of erythrocytes and erythroid precursors as a homodimer. Glycophorin A bears the antigenic determinants for the MN and Ss blood groups and has been proposed to provide a large mucin-like surface to erythrocytes that acts to minimize aggregation in circulation. Glycophorin A is first detectable on morphologically recognizable erythroid precursors just after the colony-forming unit erythroid (CFU-E) stage, and reaches its maximal expression at the late normoblast stage. Anti-glycophorin is useful in combination with anti-transferrin receptor (CD71) to identify distinct stages of erythroid differentiation since CD71 expression precedes Glycophorin A expression, but is lost during maturation of normoblasts into mature red blood cells (RBCs). Peptide-binding ELISA data indicate that the epitope of the 2B7 antibody is located in the extracellular domain of CD235a within the sequence Ala54 – Ser66 (AATPRAHEVSEIS).