In 4 cases of DLBCL (cases 1, 4, 5, and 6) with SOCS1 mutations, where normal tissue could be separated from the tumor by microdissection, the absence of SOCS1 mutations in the normal tissue revealed the somatic origin of the mutations in the lymphomas. All plasmacytomas were extramedullary. All anaplastic large cell lymphomas were ALK⁺ and the breast cancers HER2⁺ in immunohistochemistry. Successful sorting of naive and GC B cells by fluorescence-activated cell sorter was controlled by amplification of V_H genes from 20 naive and 20 GC B cells. Whereas all GC B cells carried somatic mutations (average mutation frequency, 8.8%; range, 3.2%-22.2%), all naive B cells were unmutated. One DLBCL carried only a mutation in the 5′ untranslated region. Of the DLBCL cell lines, OCI-LY-1 carried a silent and OCI-LY-18 a mutation in the 3′ untranslated region. The mutations in the DLBCL cell lines and primary samples with replacement mutations are shown in Figure 1 and supplemental Table 1. The DLBCL cell lines OCI-LY-3, -4, -7, -8, -10, SU-DHL-4, -5, -6, -7, -10, DB, HAT, Karpas 422, Pfeiffer, Toledo, and WSU carried no SOCS1 mutations. With the exception of Farage, all mutations were detected as double sequences in sequence electropherograms.

Length of the 5′ fragment, 305 bp; 3′ fragment, 513 bp.

Single nucleotide missense mutations and deletions. For tumor samples, mutation frequencies were calculated assuming that 2 alleles were amplified. For sequences from B cells, mutation frequencies were calculated assuming that always 1 allele was amplified. However, in 3 of 4 cells with mutations, the mutations were detected as double peaks, indicating that, probably from a considerable fraction of cells, 2 alleles were amplified. Assuming that 2 alleles were amplified from 75% of the cells, the mutation frequency for all 3 donors together would be 0.005%.