**Core Concept**
The Oncotype DX test is a genomic test used in the management of early-stage breast cancer to predict the likelihood of chemotherapy benefit and recurrence risk. It analyzes the expression of 21 genes in a tumor sample to provide a Recurrence Score (RS) that ranges from 0 to 100.

**Why the Correct Answer is Right**
The Oncotype DX test has been shown to accurately predict the benefit of chemotherapy in early-stage breast cancer. The test is particularly useful in patients with hormone receptor-positive, HER2-negative breast cancer. The Recurrence Score (RS) is calculated based on the expression of 21 genes, including proliferation genes (e.g., CCNB1, MKI67) and other genes involved in tumor biology. The RS is then used to categorize patients into low-risk (RS ≤ 10), intermediate-risk (RS 11-25), and high-risk (RS ≥ 26) groups.

**Why Each Wrong Option is Incorrect**
**Option A:** The Oncotype DX test is not limited to patients with estrogen receptor-positive (ER+) breast cancer only. While ER+ status is a common indication for the test, it can also be used in patients with other hormone receptor statuses.
**Option B:** The Oncotype DX test does not directly measure the expression of the HER2 gene. Instead, it uses a combination of 21 genes to provide a Recurrence Score.
**Option C:** The Oncotype DX test is not a diagnostic test for breast cancer, but rather a prognostic test used to predict the likelihood of recurrence and chemotherapy benefit.

**Clinical Pearl / High-Yield Fact**
The Oncotype DX test is an example of a personalized medicine approach in breast cancer management, where treatment decisions are based on individual tumor biology rather than a one-size-fits-all approach.

**Correct Answer:** D.