Cancer cells start in a milk duct, break through the walls, and invade breast tissue. It can remain localized, which means it stays near the site where the tumor started. Or cancer cells may spread anywhere in the body. Most benign tumors are not harmful to human health. Even though they are not cancerous, some may press against nerves or blood vessels and cause pain or other negative effects. The Premalignant class of tumor is difficult to distinguish. It can strongly resemble a benign tumor in its earlier phase and later on in it can look much more like a malignant tumor. Most bone sarcomas arise in apparently normal bone. However, some sarcomas arise in preexisting benign bone tumors or in non-neoplastic conditions, pertaining to tissue that is not yet malignant but is poised to become malignant. Malignant tumors are cancerous. Cells in these tumors can invade nearby tissues and spread to other parts of the body. When a tumor is suspected to be malignant, the doctor will perform a biopsy to determine the severity or aggressiveness of the tumor. Risk factors for developing breast cancer include being female, obesity, and lack of physical exercise, drinking alcohol, hormone replacement therapy during menopause, ionizing radiation, and early age at first menstruation, having children late or not at all, older age, and family history. Breast cancer most commonly develops in cells from the lining of milk ducts and the lobules that supply the ducts with milk.