FLAER (fluorescently labeled aerolysin). Aerolysin is a bacterial protein that binds selectively to GPI-APs. When used for flow cytometric analysis, leukocytes that express GPI-APs bind the fluorescently labeled reagent. PNH leukocytes do not bind FLAER because they do not express GPI-APs. Therefore, PNH leukocytes are identified in the flow cytometric histogram as a population of cells with absent or dim fluorescence. FLAER cannot be used for analysis of erythrocytes because, for unknown reasons, the reagent does not bind well to human red cells under the conditions required for the assay.