**Core Concept**
In the context of breast cancer, Her-2 neu amplification refers to the overexpression of the human epidermal growth factor receptor 2 (HER2) gene. This genetic alteration is present in approximately 20% of breast cancers and is associated with a more aggressive disease course. Fluorescence in situ hybridization (FISH) is a molecular diagnostic technique used to detect and quantify HER2 gene amplification.

**Why the Correct Answer is Right**
FISH is a suitable method for detecting HER2 gene amplification in breast cancer tissues, particularly when immunohistochemistry (IHC) results are equivocal or inconclusive. The FISH test involves the hybridization of fluorescent probes to the HER2 gene, allowing for the visualization of gene amplification at the cellular level. This technique is particularly useful in cases where IHC results are ambiguous, as it provides a more definitive diagnosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** While IHC is the primary method for detecting HER2 overexpression, FISH is not typically used as a first-line test. IHC results are usually interpretable, and FISH is reserved for cases where IHC is inconclusive.
* **Option B:** PCR (polymerase chain reaction) can be used to detect HER2 gene amplification, but it is not the preferred method in clinical practice. FISH is more widely used and accepted due to its higher sensitivity and specificity.
* **Option C:** While FISH is a molecular diagnostic technique, it is not used for detecting other genetic mutations in breast cancer. Other techniques, such as PCR or next-generation sequencing, may be used for this purpose.

**Clinical Pearl / High-Yield Fact**
When interpreting FISH results, a HER2/CEP17 ratio of ≥2.0 or 6 or more HER2 gene copies per cell indicates HER2 gene amplification. This information is crucial for guiding treatment decisions, including the use of targeted therapies such as trastuzumab.

**Correct Answer:** C.