**Core Concept**
The BRCA1 gene is a tumor suppressor gene involved in maintaining the integrity of the genome, particularly in DNA repair mechanisms. Mutations in this gene increase the risk of breast and ovarian cancer. Understanding the chromosomal location of BRCA1 is crucial for genetic counseling and risk assessment.

**Why the Correct Answer is Right**
The BRCA1 gene is located on the long arm of chromosome 17 (17q21). This location is significant because it is one of the most common sites for chromosomal deletions and translocations. The gene encodes a protein that plays a key role in repairing DNA damage through the homologous recombination pathway. When this pathway is disrupted, cells may become more susceptible to genetic mutations, leading to cancer.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because BRCA1 is not located on chromosome 13.
* **Option B:** Incorrect because BRCA1 is not located on chromosome 21.
* **Option D:** Incorrect because BRCA1 is not located on chromosome 22.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that BRCA1 mutations are associated with an increased risk of breast cancer, particularly in women of Ashkenazi Jewish descent. Additionally, BRCA1 mutations can also increase the risk of ovarian cancer, making genetic testing a valuable tool for risk assessment and family screening.

**Correct Answer: C. 17q21**