Breast cancer happens when cells in your breast grow and divide in an uncontrolled way, creating a mass of tissue called a tumor. Like other cancers, breast cancer can invade and grow into the tissue surrounding your breast. It can also travel to other parts of your body and form new tumors, causing metastasis. The etiology of breast cancer may be among the most complicated of all cancers given inherent, life-long exposures to multiple endogenous and exogenous factors. The etiology of breast cancer is strongly influenced by defects in the DNA damage repair (DDR) components, in particular to defects of homologous recombination repair (HRR). In 2020, there were 2.3 million women diagnosed with breast cancer and 685,000 deaths globally. As of the end of 2020, there were 7.8 million women alive who were diagnosed with breast cancer in the past 5 years, making it the world’s most prevalent cancer. There are more lost disability-adjusted life years (DALYs) by women to breast cancer globally than any other type of cancer.