Genetic factors: About 5-10% of breast cancers are caused by mutations in two genes called breast cancer susceptibility gene 1 (BRCA1) and breast cancer susceptibility gene 2 (BRCA2), respectively. If a person inherits these mutated genes, they are at increased risk of breast cancer. Therefore, those who have a family history of breast cancer, especially first-degree relatives (parents, sisters) who have had breast cancer, are at increased risk of developing breast cancer. Menstrual factors: Early menarche (age 5) can increase the risk of breast cancer, which is related to the duration of estrogen action in the body. Childbearing factors: Women who have not given birth and those who have given birth for the first time after age 30 have a mildly increased risk of breast cancer. The risk of breast cancer is mildly reduced in women who breastfeed themselves after childbirth. Effects of radiation: The risk of breast cancer is increased in women who have received radiation treatment to the chest during adolescence for diseases such as lymphoma. Hormone replacement therapy: Menopausal women who use hormone supplementation to relieve menopausal symptoms have an increased risk of developing breast cancer on their own (there are different views recently). Atypical breast hyperplasia: Atypical breast hyperplasia is a precancerous lesion that significantly increases the chance of developing breast cancer in the future, but common breast hyperplasia does not increase the risk of breast cancer. Breast cancer on one side: Women who have breast cancer on one side have a greater chance of having cancer in the opposite breast than normal people, so breast cancer patients must be followed up regularly for observation. Lifestyle: Women who drink alcohol for a long time have a higher risk of breast cancer. Obesity, especially postmenopausal obesity, also has a higher incidence of breast cancer than normal weight people.