The band 1q21 is among the most common sites affected by chromosomal translocations in lymphoid, myeloid, epithelial, and sarcomatous lesions. In non-Hodgkin's lymphoma (NHL), translocations and duplications affecting this chromosomal site are frequently, but not exclusively, seen in association with primary abnormalities such as the t(14;18)(q32;q21) and t(8;14)(q24;q32) translocations, suggesting a role for 1q21 rearrangements in tumor progression. We report here the characterization and cloning of breakpoints in a case of extranodal ascitic B-cell lymphoma with a t(1;14)(q21;q32) translocation. The breakpoints on the der(1) and der(14) chromosomes were mapped by fluorescence in situ hybridization and Southern blot analysis and cloned using an IGHG (Cgamma) probe. The translocation linked the IGHG4 switch (Sgamma4) sequences of the productively rearranged allele to chromosome 1 sequences downstream of MUC1, leaving the MUC1 transcriptional unit intact. MUC1 was markedly overexpressed in the tumor at the mRNA and protein levels relative to lymphoma cell lines lacking a 1q21 rearrangement. Presumably, MUC1 transcription is aberrantly regulated by the IGHA (Calpha) 3' enhancer element retained on the same chromosome. Screening of a panel of B-cell lymphomas by Southern blot analysis identified a subset with a 3' MUC1 breakpoint and another with low-level amplification of MUC1. MUC-1 mucin has previously been shown to be frequently overexpressed in human epithelial cancers and to be associated with tumor progression and poor clinical outcome. Thus, MUC1 activation by chromosomal translocation, rearrangement, and amplification, identified here for the first time in NHL, is consistent with its suggested role in tumorigenesis. (Blood. 2000;95:2666-2671)