Abstract: Although breast cancer is relatively rare in young women, it ranks first among the tumors that cause young women to die. There are significant differences in etiology and biology between young female breast cancer and elderly female breast cancer. The pathogenesis of young breast cancer may involve breast cancer susceptibility gene mutations, tumor suppressor gene mutation, rare copy number variations, abnormal mismatch repair, increased expression of stem cell-related genes, ionizing radiation, the activation of carcinogenic pathway, etc. Furthermore, the mechanism of pregnancy-associated breast cancer, a special type of breast cancer, has its unique characteristics. The formation of young breast cancer may be a complex process involving multiple genes, multiple steps and a long period of time, which is the interaction result of multiple factors. This paper reviews the previously published literatures in the aspects of molecular biology.