**Question:** Which of the following act as a tumour suppressor gene?

A. TP53
B. RAS
C. HER2
D. BRCA1

**Core Concept:**
Tumour suppressor genes are a group of genes that play a crucial role in preventing the development and progression of tumours by regulating cell growth and preventing uncontrolled cell division. These genes ensure that cells undergo apoptosis (programmed cell death) when necessary, preventing the formation of abnormal cells that may lead to cancer.

**Why the Correct Answer is Right:**
TP53 (Tumor Protein 53) is a well-known tumour suppressor gene. It plays a critical role in maintaining genomic stability, detecting DNA damage, and initiating cell cycle arrest or apoptosis to prevent uncontrolled cell growth and the development of cancer. TP53 is involved in regulating the p53 protein, which directly targets and induces apoptosis in response to DNA damage or other cellular stresses.

**Why Each Wrong Option is Incorrect:**

B. RAS (Ras proto-oncogene): RAS proteins are involved in cell signalling pathways but are not tumour suppressor genes. They are part of the RAS-RAF-MEK-ERK pathway, which regulates cell growth, differentiation, and survival. Mutations in RAS genes lead to uncontrolled cell division, promoting cancer development.

C. HER2 (Human Epidermal Growth Factor Receptor 2): HER2 is a receptor tyrosine kinase involved in cell signalling pathways, particularly the epidermal growth factor receptor (EGFR) pathway. Although HER2 is not a tumour suppressor gene, overexpression of HER2 can contribute to uncontrolled cell proliferation and is associated with certain types of cancer, particularly breast and ovarian cancers.

D. BRCA1 (Breast Cancer 1): BRCA1 is a tumor suppressor gene associated with hereditary breast and ovarian cancer. Mutations in BRCA1 increase the risk of developing these cancers due to its role in DNA repair, cell cycle regulation, and apoptosis.

**Clinical Pearl:**
Understanding the role of tumour suppressor genes and oncogenes is essential in understanding cancer development and treatment strategies. Tumour suppressor genes like TP53 help prevent cancer by regulating cell growth, differentiation, and apoptosis, while oncogenes (like RAS, HER2, and BRCA1) can contribute to cancer development when mutated or overexpressed. Recognizing these gene mutations can aid in diagnosing and managing patients with cancer, guiding treatment decisions and prognosis.