**Core Concept**
BCL-2 (B-cell lymphoma 2) is a key protein that regulates apoptosis, or programmed cell death. It acts as an anti-apoptotic factor by inhibiting the activation of caspases, a family of cysteine proteases that play a central role in the execution phase of cell death.

**Why the Correct Answer is Right**
BCL-2 is often overexpressed in various types of cancers, including follicular lymphoma, mantle cell lymphoma, and multiple myeloma, where it contributes to the resistance of cancer cells to apoptosis. This overexpression is a result of the translocation of the BCL-2 gene to the immunoglobulin heavy chain locus on chromosome 14, leading to its overexpression. The overexpression of BCL-2 can be detected using immunohistochemistry or flow cytometry.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because BCL-6 is a different protein that is involved in the regulation of B-cell differentiation and is often overexpressed in diffuse large B-cell lymphoma.
**Option B:** Incorrect because P53 is a tumor suppressor protein that regulates cell cycle and apoptosis, but it is not the marker for BCL-2 overexpression.
**Option C:** Incorrect because CD20 is a cell surface antigen expressed on B cells, but it is not directly related to BCL-2 overexpression.

**Clinical Pearl / High-Yield Fact**
BCL-2 overexpression is associated with a poor prognosis in various types of lymphomas, and its detection can be used as a marker for the diagnosis and monitoring of these diseases.

**Correct Answer: B. BCL-2 is the marker for B-cell lymphoma 2 overexpression.**