As shown, the expression of HLA-DR molecules on CD34 positive peripheral blood cells was doubled in PNH patients as compared to controls when measured as ABC (median 38500 and 18600 HLA-DR molecules per cell in patients and control, respectively; P = 0.025), suggesting that the avidity of contact between HSCs with specific T cells can be elevated in PNH patients. Here, CD34 is linked to paroxysmal nocturnal hemoglobinuria.