Variables not listed in Table 4—including alternative conditioning regimens, hematopoietic recovery status, GVHD prophylaxis, lymphocyte subpopulation counts, absolute lymphocyte counts, and serial measurements of immunoglobulins (IgM, IgA, IgG), complements (C3, C4), and blood LDH at reactivation—showed no significant association with PTLD in univariate analysis. Here, C4A is linked to post-transplant lymphoproliferative disease.