ETn (early transposon) elements are moderate repetitive sequences present in hundreds of copies in the mouse genome. Their length ranges from 4.4 to 7.1 kb, and, like transposons, they contain long terminal repeats (LTRs) on both sides and are flanked by target site duplications (Kaghad et al. 1985). ETn-related elements can be grouped into three distinct families. Members of the ETn I and ETn II families mainly contain sequences of unknown origin in their core region. Only very short stretches of retrovirus-like sequences are present, and there are no ORFs. ETn I and ETn II elements differ primarily in the 3- half of both the 5- and 3- LTR, and in the 5- end of the core region (see Fig. 1). As a consequence, only ETn II elements contain a primer binding site for tRNALys. In contrast to ETn I and ETn II, members of the recently described MusD family (Mager and Freeman 2000) contain ORFs for (at least parts of) D-type virus Gag, Pro, and Pol proteins. However, in other regions they are structurally similar to ETn II elements and contain an intact primer binding site. It has been shown that MusD sequences are evolutionarily older than ETn II elements, suggesting that the latter might have arisen by recombinatory replacement of the MusD gene-coding sequences with sequences of unknown origin (Mager and Freeman 2000). ETn elements are still active as retrotransposons. In the past years, several germ line and somatic mutations caused by fresh ETn integrations have been found (Table 1). From 19 mutations, sufficient sequence is available in seven cases to show that the insertion was an ETn II element. In eight cases, the sequence data available indicate either an ETn II or a MusD element. ETn I has not been found to be the cause of any mutations, prompting the suggestion that ETn II is the "mobile" family, whereas ETn I elements have lost the capacity to retrotranspose.