An example of a tumour suppressor gene is

**Core Concept**
Tumour suppressor genes are a class of genes that regulate cell growth and division by preventing the uncontrolled proliferation of cells, thereby preventing cancer. These genes encode proteins that either directly or indirectly inhibit cell growth, induce apoptosis, or repair DNA damage.

**Why the Correct Answer is Right**
The correct tumour suppressor gene is p53, which is often referred to as the "guardian of the genome." The p53 protein plays a crucial role in maintaining genomic stability by arresting the cell cycle in response to DNA damage, allowing for repair or inducing apoptosis if the damage is irreparable. This is achieved through its ability to bind to specific DNA sequences and activate the transcription of target genes involved in cell cycle arrest, DNA repair, and apoptosis.

**Why Each Wrong Option is Incorrect**
**Option A:** Rb (Retinoblastoma protein) is a tumour suppressor gene, but it is not the correct answer. Rb regulates cell cycle progression by inhibiting the E2F family of transcription factors, which are involved in promoting cell growth and proliferation.

**Option B:** No option is provided for B, so we will skip it.

**Option C:** No option is provided for C, so we will skip it.

**Clinical Pearl / High-Yield Fact**
A key feature of tumour suppressor genes is their ability to be inactivated by mutations, leading to uncontrolled cell growth and cancer. The p53 gene is the most commonly mutated gene in human cancers, highlighting its critical role in preventing cancer development.

**Correct Answer: A. p53**