A family of functional neogenes called Mart, related to the gag gene of Sushi-like long terminal repeat retrotransposons from fish and amphibians, is present in the genome of human (11 genes) and other primates, as well as in mouse (11 genes), rat, dog (12 genes), cat, and cow. Mart genes have lost their capacity of retrotransposition through non-functionalizing rearrangements having principally affected long terminal repeats and pol open reading frame. Most Mart genes are located on the X chromosome in different mammals. Sequence database analysis suggested that Mart genes are present in opossum (marsupial), but absent from the genome of chicken. Hence, the Mart gene family might have been formed from Sushi-like retrotransposon(s) after the split of birds and mammals (310 myr ago), but before the divergence between placental mammals and marsupials (170 myr ago). RT-PCR analysis showed that at least six Mart genes are expressed during mouse embryonic development, with in situ hybridization analysis revealing rather ubiquitous expression patterns. Mart expression was also detected in adult mice, with some genes being expressed in all tissues tested, while others showed a much more restricted expression pattern. Although additional analysis will be required to establish the function of the retrotransposon-derived Mart neogenes, these observations support the evolutionary importance of retrotransposable elements as a source of genetic novelty.