A method of diagnosing metastatic potential of a breast cancer in an individual with breast cancer is described. The method comprises a step of assaying a breast cancer tumor sample from the patient for expression of A Disintegrin And Metalloproteinase 22, (ADAM22), wherein expression of ADAM22 correlates with increased potential for metastasis compared with a patient who is ADAM22 negative. The invention also describes an agent for use in the treatment of metastatic breast cancer in a patient, in which the agent is selected from leucine-rich, glioma-inactivated protein 1 (LGI1) protein (SEQ ID NO:1) and an LGI1 peptide mimic capable of mimicking the ADAM22 binding activity of LGI1 by binding to the LGI1 binding domain of ADAM22 (SEQ ID NO: 4) and which is capable of inhibiting migration of endocrine resistant breast cancer cells.
