**Core Concept**
Odontoblastic differentiation is a complex process involving multiple genetic and molecular mechanisms. The process of tooth development and dentin formation is tightly regulated by various transcription factors and signaling pathways. The correct answer involves a specific gene that plays a crucial role in the differentiation of odontoblasts.

**Why the Correct Answer is Right**
The gene involved in odontoblastic differentiation is the _MSX2_ gene. MSX2 is a member of the muscle segment homeobox gene family and is known to play a critical role in the development of teeth and other ectodermal tissues. The MSX2 protein acts as a transcription factor, regulating the expression of other genes involved in odontoblastic differentiation. Specifically, MSX2 promotes the expression of genes involved in dentin formation, such as the dentin sialophosphoprotein (DSPP) gene.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because while the _PAX9_ gene is involved in tooth development, it is not specifically involved in odontoblastic differentiation.

**Option B:** This is incorrect because the _EN1_ gene is involved in the development of the enamel organ and the formation of enamel, but not specifically in odontoblastic differentiation.

**Option C:** This is incorrect because the _SHH_ gene is involved in the regulation of tooth development and patterning, but not specifically in odontoblastic differentiation.

**Clinical Pearl / High-Yield Fact**
The MSX2 gene is a key regulator of tooth development and dentin formation. Understanding the role of MSX2 in odontoblastic differentiation is crucial for the diagnosis and treatment of various dental disorders, including dentinogenesis imperfecta and dentin dysplasia.

**Correct Answer: D. MSX2**