Once a cancer-causing mutation does occur, the affected cell, which is most commonly an epithelial cell that lines the ducts or the lobules, begins to grow and replicate out of control, forming a tumor.

一旦致癌突变发生，受损的细胞，通常是导管或小叶内的上皮细胞会开始失控地增长复制，形成肿瘤。