## **Core Concept**
MODY (Maturity-Onset Diabetes of the Young) refers to a group of monogenic disorders characterized by autosomal dominant inheritance, onset typically before the age of 25, and a primary defect in pancreatic beta-cell function. MODY 1 is a specific subtype of MODY.

## **Why the Correct Answer is Right**
MODY 1 is caused by mutations in the **HNF4A** gene. The HNF4A gene encodes hepatocyte nuclear factor 4 alpha, a transcription factor crucial for the regulation of glucose-stimulated insulin secretion in pancreatic beta cells. Mutations in HNF4A lead to impaired insulin secretion, resulting in hyperglycemia.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While there are several genes associated with MODY, mutations in the **HNF1A** gene are associated with MODY 3, not MODY 1.
- **Option B:** Mutations in the **GCK** gene are associated with MODY 2, which is a different subtype of MODY.
- **Option D:** **HNF1B** mutations are associated with MODY 5, another subtype of MODY.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that MODY 1, caused by **HNF4A** mutations, often presents with a family history of diabetes and may be confused with type 1 or type 2 diabetes due to its variable presentation. It's essential to consider genetic testing for MODY in young patients with diabetes and a strong family history.

## **Correct Answer:** . **HNF4A**.