Cancer is a prevalent and poorly understood disease in human populations. It is generally viewed as a complex, genetic, multistep process involving a series of independent events, each of which creates an incremental phenotypic aberration. For example, the capabilities for extended proliferation, invasion of adjacent tissue, and distant metastasis might each be acquired independently by a cancer cell (1 -4 ). The molecular basis underlying the ability of tumor cells to metastasize from the primary site of growth to other tissues is a major challenge in understanding oncogenesis.
