Chromosome translocations involving the immunoglobulin heavy chain (IGH) gene locus at chromosome region 14q32 are often observed in B-cell lymphoid neoplasms. Of these, t(14;18)(q32;q21) results in juxtaposition of the IGH gene on chromosome 14 and the BCL2 gene on chromosome 18, leading to the overexpression of BCL2 anti-apoptotic protein, which plays a critical role in the development of follicular lymphoma (FL). However, BCL2 overexpression is not observed in approximately 10 % of FL, and the molecular pathogenesis of BCL2-negative FL has not been elucidated. Here, we identify the SRY-related high-morbidity-group (HMG) box 5 (SOX5) gene on chromosome 12p12 as a novel IGH-involved translocation partner in the case of BCL2-negative follicular lymphoma (FL) with a complex karyotype including t(12;14)(p12.2;q32) by long-distance inverse PCR. As a result of this translocation, the SOX5 gene is juxtaposed to the enhancer of the IGH gene; SOX5 overexpression in neoplastic cells was demonstrated by immunohistochemistry. The results of the present study suggest a role for SOX5 in the molecular pathogenesis of FL.