The invention concerns composititions and methods for the diagnosis and treatment of neoplastic cell growth and proliferation in mammals, including humans. The invention is based on the identification of cardiotrophin-1 gene amplified in the genome of tumor cells. Such gene amplification is expected to be associated with the overexpression of the gene product and contribute to tumorigenesis. Accordingly, the cardiotrophin-1 polypeptide encoded by the amplified gene is believed to be a useful target for the diagnosis and/or treatment (including prevention) of certain cancers, and may act as a predictor of the prognosis of tumor treatment.