Transfection experiments indicate that a number of genes can induce or suppress the metastatic phenotype. These genes include ten oncogenes, plasminogen activator, and three suppressor genes. The biochemical mechanism(s) whereby each of these genes alters the metastatic phenotype is poorly understood. Hypotheses concerning the coordinate regulation of metastasis-effector genes, and the relationship of gene expression in metastasis to that in embryonic development are discussed.