Flow cytometric analysis of red cells in PNH using a combination of anti-CD59 FITC, anti-CD55 PE, and anti-CD235a CyChrome. (Ai-ii) Flow cytometry plots show the gating strategy for identifying a pure population of red cells. An initial region (R1) is set on CD235a⁺FSC^High events. A second region (R2) is then drawn around the _logFSC/SSC characteristics of these cells. Only events that meet both these criteria are analyzed. (Aiii) CD59 and CD55 bivariate expression on these erythroid events. A small population of complete deficiency cells (type III) are detected (region R3 = 8.0%). (Aiv) Histogram analysis of CD59 expression also shows a major population of partially deficient cells (type II) comprising 53.9% of the total. Type I (normal expression) cells comprise 38.1% of the total. (Bi-iii) Detection of a very small red cell clone in a patient with PNH-sc/aplastic anemia. The red cell PNH clone comprises 0.4% of the total red cells (Bi). Confidence that this population is not an artifact is shown by simultaneous CD59 (Bii) and CD55 (Biii) deficiency and strong expression of CD235a. Illustration enhanced by A. Y. Chen.