**Core Concept**
Rett syndrome is a rare genetic disorder that primarily affects girls, characterized by severe mental and physical disability. It is caused by mutations in a specific gene that encodes for a protein involved in brain development.

**Why the Correct Answer is Right**
The correct answer is associated with the gene MECP2, which encodes for the methyl-CpG-binding protein 2 (MeCP2). MeCP2 plays a crucial role in brain development and function by regulating gene expression. In Rett syndrome, mutations in the MECP2 gene lead to a deficiency of MeCP2 protein, resulting in abnormal brain development and the characteristic symptoms of the disorder. This deficiency affects the normal functioning of neurons and leads to the cognitive, motor, and behavioral impairments seen in Rett syndrome.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because mutations in the UBE3A gene are associated with Angelman syndrome, a different neurodevelopmental disorder.
* **Option B:** This option is incorrect because mutations in the FMR1 gene are associated with Fragile X syndrome, another genetic disorder that affects brain development and function.
* **Option D:** This option is incorrect because mutations in the SMN1 gene are associated with spinal muscular atrophy, a genetic disorder that affects motor neuron function.

**Clinical Pearl / High-Yield Fact**
Rett syndrome is a X-linked dominant disorder, meaning that mutations in the MECP2 gene are located on the X chromosome and can be inherited from an affected mother or occur spontaneously.

**Correct Answer: C. MECP2 protein is associated with Rett syndrome.**