Abstract: An embodiment of the present invention provides a marker, a test method, and a test kit which can detect the onset of breast cancer that cannot be detected by palpation or mammography examination or breast cancer in an early stage (clinical stage 0), which are simple, and which have high reliability. A marker associated with breast cancer of an embodiment of the present invention is characterized by being a micro-RNA that is found in serum or plasma. More specifically, the marker contains at least a micro-RNA that is present in the serum or the plasma at a significantly reduced level after the onset of breast cancer, or during or after an early stage (during or after clinical stage 0) of breast cancer compared with that before the onset of breast cancer or before the early stage (before clinical stage 0) of breast cancer.

Abstract: An embodiment of the present invention provides a marker, a test method, and a test kit which can detect the onset of breast cancer that cannot be detected by palpation or mammography examination or breast cancer in an early stage (clinical stage 0), which are simple, and which have high reliability. A marker associated with breast cancer of an embodiment of the present invention is characterized by being a micro-RNA that is found in serum or plasma. More specifically, the marker contains at least a micro-RNA that is present in the serum or the plasma at a significantly reduced level after the onset of breast cancer, or during or after an early stage (during or after clinical stage 0) of breast cancer compared with that before the onset of breast cancer or before the early stage (before clinical stage 0) of breast cancer.