Clinical manifestations of PNH are determined by clone size and erythrocyte phenotype. Mock flow cytometry histograms of erythrocytes from hypothetical patients with PNH stained with anti-CD59 are illustrated. The proportion and type of abnormal erythrocytes varies greatly among patients with PNH, and these characteristics are important determinants of clinical manifestations. Patients with a high percentage of type III erythrocytes have clinically apparent hemolysis (A). If the erythrocytes are partially deficient (∼ 10% of normal expression) in GPI-AP (PNH II cells), hemolysis may be modest even if the percentage of the affected cells is high (B). A patient may have a diagnosis of PNH, but if the proportion of type III cells is low, only biochemical evidence of hemolysis may be observed (C).