Mantle cell lymphoma (MCL) accounts for 5% to 10% of mature B-cell neoplasms and is characterized by overexpression of cyclin D1 (bcl-1) protein due to the specific translocation t(11;14) (q13;q32). However, approximately 5%-10% of MCLs lack cyclin D1 expression and may be misdiagnosed by overreliance on cyclin D1 IHC. Recent studies showed that the strong nuclear expression SOX-11 was found in almost all cyclin D1-positive MCL (93%-100%) and cyclin D1-negative MCL (~100%). The nuclear expression of SOX-11 is highly associated with both cyclin D1-positive and negative MCL. SOX-11 IHC is of value in further defining pathologic features of CD5+ DLBCL. Routine use of anti-SOX-11 in cases of suspected CD5+ DLBCL might help identify additional cases of cyclin D1-negative blastoid MCL. SOX-11 can also be detected in some BL, LBL and T-PLL, although the different morphological and phenotypic features of these malignancies allow easy recognition of the cases of cyclin D1-negative MCL.