AbstractHuman leukocyte antigen (HLA) class I molecules are expressed on the surface of all nucleated cells and present antigenic peptides to cytotoxic T cells, thereby playing an important role in initiating the cellular anti‐tumor immune response. We previously reported that loss of HLA class I expression in diffuse large B‐cell lymphoma (DLBCL) of the central nervous system (CNS) and the testis is a common event. Loss of expression and mutations of the light chain of the HLA class I molecule, β2‐microglobulin (β2m) have been described in a variety of human tumors and cell lines. In our study, we screened 15 DLBCL cases with a combined loss of HLA class I and β2m expression for mutations in the latter gene by direct sequencing. Frame shift mutations in repetitive sequences within the β2m gene leading to loss of functional β2m were detected in 2 cases. Loss of heterozygosity (LOH) and fluorescent in situ hybridization (FISH) analysis for chromosome 15 exhibited loss of the remaining copy of the β2m gene in both cases but also hemizygous deletions and monosomies in 6 additional cases. Since similar mutations in the β2m gene have been associated with microsatellite instability (MSI), we used 8 markers to study MSI involvement in DLBCL. Low MSI was more frequent (33%) as compared to nodal DLBCL (n=15) but did not correlate with the β2m mutations. Our data indicate that multiple mechanisms lead to downregulation of β2m and concomitant loss of HLA class I expression in DLBCL. © 2002 Wiley‐Liss, Inc.