T-cell–rich B-cell lymphoma (TCRBCL) belongs to the group of diffuse large cell lymphomas (DLL). It is characterized by a small number of tumor B cells among a major population of nonmalignant polyclonal T cells. To identify the developmental stage of the tumor progenitor cells, we micromanipulated the putative neoplastic large CD20⁺ cells from TCRBCLs and amplified and sequenced immunoglobulin (Ig) V gene rearrangements from individual cells. In six cases, clonal Ig heavy, as well as light chain, gene rearrangements were amplified from the isolated B cells. All six cases harbored somatically mutated V gene rearrangements with an average mutation frequency of 15.5% for heavy (V_H) and 5.9% for light (V_L) chains and intraclonal diversity based on somatic mutation. These findings identify germinal center (GC) B cells as the precursors of the transformed B cells in TCRBCL. The study also exemplifies various means how Ig gene rearrangements can be modified by GC B cells or their malignant counterparts in TCRBCL: In one case, the tumor precursor may have switched from κ to λ light chain expression after acquiring a crippling mutation within the initially functional κ light chain gene. In another case, the tumor cells harbor two in-frame V_H gene rearrangements, one of which was rendered nonfunctional by somatic mutation. Either the tumor cell precursor entered the GC with two potentially functional in-frame rearrangements or the second V_HD_HJ_Hrearrangement occurred in the GC after the initial in-frame rearrangement was inactivated by somatic mutation. Finally, in each of the six cases, at least one cell contained two (or more) copies of a clonal Ig gene rearrangement with sequence variations between these copies. The presence of sequence variants for V region genes within single B cells has so far not been observed in any other normal or transformed B lymphocyte. Fluorescence in situ hybridization (FISH) points to a generalized polyploidy of the tumor cells.