A method of treating a tumor (in particular osteosarcoma or Ewing's sarcoma) in a host by administering to a host or to the tumor cells an agent which inhibits cyclin G1 protein in an amount effective to inhibit cyclin G1 protein in tumor cells of the host. The agent may be an antisense polynucleotide which is complementary to at least a portion of a polynucleotide encoding cyclin G1 protein, or an antibody or fragment or derivative thereof which recognizes cyclin G1 protein. Also contemplated within the scope of the present invention are (i) the immortalization of cell lines by transducing cells with a polynucleotide encoding cyclin G1 protein; (ii) increasing the receptiveness of cells to retroviral infection by transducing cells with a polynucleotide encoding cyclin G1 protein; and (iii) the detection of cancer by detecting cyclin G1 protein or a polynucleotide encoding cyclin G1 protein in cells. In addition, the present invention provides expression vehicles, such as, for example, retroviral vectors and adenoviral vectors, which include polynucleotides which encode agents which inhibit cyclin G1 protein, and expression vehicles which include a polynucleotide encoding cyclin G1 protein.