A method of diagnosing cancer comprising the steps of (i) obtaining a sample containing nucleic acid and/or protein from the patient; and (ii) determining whether the sample contains a level of SCN5A (and optionally also SCN9A) voltage-gated Na+ channel nucleic acid or protein associated with cancer. A method of diagnosing breast cancer comprising the steps of (i) obtaining a sample containing nucleic acid and/or protein from the patient; and (ii) determining whether the sample contains a level of voltage-gated Na+ channel nucleic acid or protein, preferably SCN5A or 5CN9A, associated with cancer. A method of treating cancer comprising the step of administering to the patient an agent which selectively prevents the function of SCN5A (and optionally also SCN9A) voltage-gated Na+ channel. A method of treating breast cancer comprising the step of administering to the patient an agent which selectively prevents the function of a voltage-gated Na+ channel, preferably SCN5A or 5CN9A. Genetic constructs and molecules useful in such methods. The methods and compositions are particularly suited to breast cancer.