The subject invention pertains to tumor cell lines useful for increasing the proliferation potential of any human or animal cell in culture, thereby providing immortalized or continuous cell lines and cultures. The invention also concerns proliferation factors, and compositions containing the factors, which are capable of increasing the proliferation potential of any human or other animal cell in culture. The subject invention further pertains to a method for proliferating cells in culture by containing cells with the proliferation factors. The proliferated cells can range in plasticity and can include, for example, blast cells, fertilized ova, non-fertilized gametes, embryonic stem cells, adult stem cells, precursor or progenitor cells, and highly specialized cells. Optionally, the cells can be induced to cease proliferation. The proliferated cells of the subject invention are useful for cell therapy, cell/gene therapy, biological production of molecules, and as in vitro models for research, toxicity testing, and drug development.