A prognostic assay and kit and method of use thereof are provided. The kit and assay are used to determine the likelihood of a diseased cell or tissue having a therapeutic response to treatment with a cardiac glycoside in a disease having an etiology associated with excessive cell proliferation. The kit and assay are used to determine the ratio of isoforms of the α subunit of Na, K-ATPase obtained from the diseased cell or tissue. The kit can be used to predict the therapeutic responsiveness of cancer or tumor in a subject to treatment with a cardiac glycoside. The kit and assay can be incorporated in a method of treating a disease or disorder having an etiology associated with excessive cell proliferation with a composition comprising a cardiac glycoside.