The molecular mechanisms potentially responsible for cadmium-induced cell transformation and tumorigenesis were investigated using Balb/c-3T3 cells transformed with cadmium chloride. Differential display analysis of gene expression revealed consistent and reproducible overexpression of a transcript in the transformed cells compared with the nontransformed cells. The full-length cDNA corresponding to the differentially expressed transcript was cloned and was identified as mouse translation elongation factor-1 delta subunit (TEF-1 delta; GenBank accession number ). Nucleotide sequence analysis of TEF-1 delta cDNA revealed an open reading frame encoding the predicted protein of 281 amino acids and exhibited significant conservation with the corresponding protein of human, Xenopus laevis, and Artemia. The presence of a leucine zipper motif, characteristic of translation elongation factor-1 delta, was also found in the mouse TEF-1 delta. A 31-kDa protein was detected in eukaryotic cells transfected with an expression vector containing the TEF-1 delta cDNA. Overexpression of the TEF-1 delta protein by transfection was oncogenic in NIH3T3 cells as evidenced by the appearance of transformed foci exhibiting anchorage-independent growth and the potential to grow as tumors in nude mice. Blocking the translation of TEF-1 delta with antisense TEF-1 delta mRNA resulted in a significant reversal of the oncogenic potential of cadmium-transformed Balb/c-3T3 cells as evidenced from suppression in anchorage-independent growth and tumorigenesis in nude mice. Our findings demonstrate, for the first time, that the cell transformation and tumorigenesis induced by cadmium are due, at least in part, to the overexpression of TEF-1 delta, a novel cadmium-responsive proto-oncogene.