**Core Concept**
Mantle cell lymphomas (MCL) are a subtype of B-cell non-Hodgkin lymphoma characterized by the translocation t(11;14)(q13;q32), leading to the overexpression of cyclin D1. This genetic abnormality is a hallmark of MCL and contributes to its pathogenesis.

**Why the Correct Answer is Right**
Mantle cell lymphomas are typically positive for cyclin D1 due to the t(11;14) translocation, which results in the juxtaposition of the cyclin D1 gene (CCND1) with the immunoglobulin heavy chain gene (IGH) locus. This leads to the overexpression of cyclin D1, a cell cycle regulator that promotes cell proliferation. The overexpression of cyclin D1 is a key feature of MCL and is used as a diagnostic criterion.

**Why Each Wrong Option is Incorrect**
**Option A:** Mantle cell lymphomas are often positive for CD5, a cell surface protein that is also expressed on T cells. While CD5 is a useful marker for distinguishing MCL from other types of lymphoma, it is not exclusive to MCL and can be expressed on other lymphoid neoplasms.
**Option B:** Mantle cell lymphomas are typically negative for CD23, a marker that is often expressed on chronic lymphocytic leukemia/small lymphocytic lymphoma (CLL/SLL). In contrast, MCL is usually CD23-negative.
**Option C:** Mantle cell lymphomas are often positive for CD19 and CD20, which are pan-B-cell markers. These markers are useful for identifying B-cell lineage and can help distinguish MCL from other types of lymphoma.

**Clinical Pearl / High-Yield Fact**
Mantle cell lymphomas are often associated with a poorer prognosis than other types of non-Hodgkin lymphoma, particularly if diagnosed at an advanced stage. Early recognition and treatment of MCL are critical for improving outcomes.

**Correct Answer: B. CD23**