A test that is used to help predict whether breast cancer will spread to other parts of the body or come back. The test looks at the activity of 21 different genes in breast cancer tissue of women who have early-stage breast cancer that is estrogen receptor positive and has not spread to the lymph nodes. If there is a high risk that the cancer will spread or come back, it may be used to help plan treatment with anticancer drugs. Also called 21-gene signature.