Specificity of protein sorting into exosomes during reticulocyte maturation. Reticulocytes (Retics; 10⁶ cells; left lanes) and exosomes (Exos.; 50 μg protein; right lanes) were assessed for the presence of transmembrane (A), cytoskeletal (B), or cytosolic (C) proteins by SDS-PAGE and immunoblotting for the proteins, as indicated on the right.