Methods, assays, and kits for determining a cancer's (e.g., breast cancer) metastatic potential and tumor aggressiveness in a subject (e.g., a human patient) and for measuring a subject's response to cancer therapy involve analyzing expression of Sema7A in a biological sample from the subject, and correlating increased expression of Sema7A in the biological sample compared to a control sample with metastatic potential of the cancer, wherein the expression of Sema7A is linearly proportional to the metastatic potential of the cancer in the subject. These methods, kits and assays provide for individualized diagnosis and treatment options for cancer (e.g., breast cancer) patients. They can be used independently, or can be combined with additional diagnostic tests and/or prognostic methods. Compositions, kits and methods for treating a subject having cancer (e.g., breast cancer) include administering a composition for inhibiting Sema7A expression or activity to the subject.