**Core Concept**
Cleidocranial dysplasia is a rare genetic disorder characterized by skeletal abnormalities, including hypoplastic or absent clavicles, delayed closure of cranial sutures, and dental anomalies. This condition is caused by mutations in a specific gene involved in bone development and growth.

**Why the Correct Answer is Right**
The correct answer is due to a defect in the _RUNX2_ gene, which encodes a transcription factor essential for osteoblast differentiation and bone formation. Mutations in _RUNX2_ disrupt the normal development of bones, leading to the characteristic features of cleidocranial dysplasia. The _RUNX2_ gene plays a crucial role in the regulation of osteoblast-specific gene expression, including the expression of alkaline phosphatase and collagen type I.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a well-established gene associated with cleidocranial dysplasia.
**Option B:** This option is incorrect because it is a gene involved in a different genetic disorder.
**Option C:** This option is incorrect because it is a gene related to a different aspect of bone metabolism.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that cleidocranial dysplasia is a rare but distinctive disorder that often presents with a combination of skeletal and dental abnormalities. A high index of suspicion is necessary for early diagnosis and management.

**Correct Answer: C. RUNX2 gene**