The gold standard for the modern diagnosis of PNH is the flow cytometric fluorescein-labeled aerolysin assay (FLAER), which uses proaerolysin with Alexa Fluor 488 to determine the percentage of normal (type I), partially GPI-deficient (type II), and fully GPI-deficient (type III) cells. Here, GPI is linked to paroxysmal nocturnal hemoglobinuria.