Hormones increase breast growth during puberty. Increased estrogen levels are also experienced in adult females during menstruation and pregnancy. Hormonal increases cause cycles of cell proliferation that increases the risk for DNA damage. If premalignant or malignant cells are present during these cycles, the hormones stimulate the growth of the malignant cells along with the normal cells. Approximately 75% of breast tumors are estrogen dependent. Tumor cells which contain estrogen receptors are called estrogen receptor (ER)-positive, while tumors which lack estrogen receptors are called ER-negative or estrogen independent.