The MCD cluster (8% of DLBCL), similar to the C5, contains almost exclusively ABC-DLBCL with aberrant activation of the chronic BCR and NF-κB signaling (mutations of MYD88, CD79A, CD79B, and CARD11), impaired terminal B cell differentiation (PRDM1 mutations or deletions, SPIB gains or amplifications), deregulated cell cycle (CDKN2A/B deletions), and immune escape (mutations or deletions of HLA-A, HLA-B, HLA-C, and CD58). The gene discussed is CD79B; the disease is aneurysmal bone cyst.