Final answer:
The BRCA1 gene was the first discovered gene associated with breast cancer development. Along with BRCA2, it increases the risk for breast and ovarian cancer when mutated.
Step-by-step explanation:
The first gene discovered to be associated with the development of breast cancer is the BRCA1 gene. This discovery was a significant step in understanding inherited risk factors for breast and ovarian cancers. The BRCA2 gene was identified subsequently and both genes are linked to a high risk of developing these cancers when mutated. Women who have a family history of these cancers can undergo genetic screening to determine if they carry these mutations. If they are carriers, preventive measures such as prophylactic surgery can be considered to reduce their risk of cancer. However, it is important to note that not every person who inherits these genes will develop cancer; environmental factors and other genetic variables also play a role.
Aside from BRCA1 and BRCA2, researchers have identified other genetic mutations associated with breast cancer, such as those in the TP53 gene, which can lead to Li-Fraumeni syndrome with a near 100% lifetime risk of developing various cancers. Moreover, oncogenes like HER2 are involved in the development of around 20% of human breast cancers and have led to targeted therapies, such as the drug Herceptin (trastuzumab), which improves survival rates for patients with metastatic disease.