## **Core Concept**
The question pertains to the genetic basis of familial gastric cancer, specifically focusing on the tumor suppressor gene (TSG) that is mutated in this condition. Familial gastric cancer, also known as hereditary diffuse gastric cancer (HDGC), is a rare inherited condition that significantly increases the risk of developing gastric (stomach) cancer.

## **Why the Correct Answer is Right**
The correct answer, **CDH1**, is the gene that encodes for E-cadherin, a protein crucial for cell-cell adhesion. Mutations in the **CDH1** gene lead to the loss of function of E-cadherin, which is a key event in the development of familial gastric cancer. This loss disrupts normal cell adhesion, leading to uncontrolled cell growth and cancer. The **CDH1** gene acts as a tumor suppressor gene, and its mutation is associated with hereditary diffuse gastric cancer.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While **TP53** is a well-known tumor suppressor gene mutated in many types of cancers (e.g., Li-Fraumeni syndrome), it is not specifically associated with familial gastric cancer.
- **Option B:** **BRCA2** is associated with increased risks of breast, ovarian, and other cancers, but it is not the gene primarily implicated in familial gastric cancer.
- **Option D:** **APC** is famously associated with familial adenomatous polyposis (FAP), a condition leading to colorectal cancer, not specifically with familial gastric cancer.

## **Clinical Pearl / High-Yield Fact**
A critical clinical pearl is that individuals with a **CDH1** mutation should undergo prophylactic gastrectomy or endoscopic surveillance due to the high risk of developing diffuse gastric cancer, which can be difficult to detect by endoscopy.

## **Correct Answer:** . **CDH1**