Mantle cell lymphoma (MCL) is characterized by the presence of t(11;14)(q13;q32) which juxtaposes CCND-1 gene (also known as BCL-1, PRAD-1) at 11q13 with an enhancer of the IGH@ gene at 14q32. The resultant overexpression of cyclin D1 plays an essential role in the pathogenesis of MCL. The breakpoints on chromosome 14 occur 5' to one of six JH segments, whereas only 30-50% of the breakpoints on chromosome 11 are localized within a 1 kb region called the major translocation cluster (MTC) which can be easily assessed by polymerase chain reaction (PCR). The remainder of the breakpoints are widely scattered over approximately 120 kb, making PCR analysis infeasible. We describe a TaqMan-based real-time PCR assay to detect and quantify IGH@/BCL1 fusion products in newly diagnosed MCL, and to monitor minimal residual disease during treatment or early relapse in MTC-positive cases.