The t(11;14) (q13;q32) translocation and its molecular counterpart Bcl-1 rearrangement are consistent features of mantle cell lymphoma (MCL). This translocation activates the PRAD1/cyclin D1 gene that is considered to be the Bcl-1 oncogene. PRAD1/cyclin D1 gene overexpression is closely associated with MCL. The PRAD1/cyclin D1 protein is localized to the nucleus, and the strong correlation between PRAD1/cyclin D1 and MCL is also found in the protein level. This finding indicates that PRAD1/cyclin D1 expression is a highly specific and sensitive molecular marker for MCL. This gene may function as an oncogene in the malignant transformation of cells.