PDCD1 and Nijmegen breakage syndrome: PD1 as a marker of exhaustion was detected on a significantly higher proportion of both CD4+ and CD8+ lymphocytes in NBS patients than in healthy controls, with median CD4+PD1+ lymphocytes composing 10.7 vs. 2.6% (p < 0.01) and CD8+PD1+ composing 7.3 vs. 3.3% (p < 0.01).