A rare disease merits general attention if it is associatedwith extraordinary clinical manifestations, unusual laboratoryfindings, or an instructive molecular defect. Paroxysmal nocturnalhemoglobinuria (PNH) has all three. Its clinical hallmark, blackurine on arising from sleep, is graphic testimony to intravascularhemolysis during the night. Even more peculiar is the hemolysisthat occurs after blood from a patient with PNH is mixed withacidified serum or ordinary table sugar. The molecular abnormalityin PNH, described by Miyata et al. in this issue of the Journal(see pages 249 to 255), not only informs us about this unusualdisease . . . [Full Text of this Article]