**Core Concept**
The BCR-ABL fusion gene is a result of a chromosomal abnormality known as the Philadelphia chromosome, which is a reciprocal translocation between chromosomes 9 and 22. This genetic mutation is a hallmark of chronic myeloid leukemia (CML) and some cases of acute lymphoblastic leukemia (ALL).

**Why the Correct Answer is Right**
The BCR-ABL fusion gene is detected using a polymerase chain reaction (PCR) test, specifically the real-time quantitative PCR (RQ-PCR). This test amplifies the BCR-ABL mRNA and measures its expression levels. The RQ-PCR test is highly sensitive and specific, allowing for the detection of even low levels of BCR-ABL expression.

*Option A:* FISH (Fluorescence In Situ Hybridization) is a technique used to detect chromosomal abnormalities, but it is not the primary method for detecting the BCR-ABL fusion gene.
*Option B:* Cytogenetics involves studying the structure and number of chromosomes, but it is not the most efficient method for detecting the BCR-ABL fusion gene.
*Option C:* Immunohistochemistry is a technique used to detect specific proteins in tissue samples, but it is not the primary method for detecting the BCR-ABL fusion gene.

**Clinical Pearl / High-Yield Fact**
The BCR-ABL fusion gene is a predictive marker for response to tyrosine kinase inhibitors (TKIs), such as imatinib, which are a mainstay of treatment for CML and some cases of ALL.

**Correct Answer:** C.