Internal fragments of the putative transposase gene of mariner-like elements (MLEs) were amplified from human, mouse, rat, chinese hamster, sheep and bovine genomic DNAs by polymerase chain reaction (PCR). The sequences identified in human, ovine and bovine genomes correspond to ancient degenerate transposons. Screening mammalian sequence libraries identified a truncated element in the human ABL gene and the sequence of its 5'-ITR was determined. This ITR sequences were used in PCR experiments with DNA from six mammalian species and detected full-sized and deleted MLEs. The presence of MLE in mammalian genomes demonstrates that they are ubiquitous mobile elements found from fungi to man. This observation strongly raises the possibility that MLE could constitute tools for the modification of eucaryotic genomes.