The synthetic L1 retrotransposon, ORFeus, is useful for probing mechanisms of L1 retrotransposition in vivo and for genome-wide mouse mutagenesis because of its high level of activity. To achieve controlled activation of ORFeus in mice, we constructed ORFeus(LSL), in which ORFeus coding sequences were separated from the promoter by a loxP-beta-geo-stop-loxP (LSL) cassette, and derived transgenic mouse lines containing single-copy ORFeus(LSL). We observed tissue-specific ORFeus activation by crossing ORFeus(LSL) to various Cre-expressing lines, specifically in the germ line or the pancreas, providing definite evidence that all host factors and machinery required posttranscriptionally for L1 retrotransposition are available in somatic tissues in living animals. Notably, the single-copy ORFeus transgene is about threefold more active per copy than a previously described multicopy ORFeus transgene in the germ line and even more active somatically. This conditional transgenic ORFeus mouse model should allow further exploration of posttranscriptional cellular requirements for L1 retrotransposition and facilitate the development of ORFeus mouse lines suitable for in vivo mutagenesis.