The combination of rad mutations and regulated HO endonuclease has proven to be a potent system for elucidating DNA double-strand break (DSB) repair mechanisms in Saccharomyces cerevisiae (see Chapter 32). An analogous system comprising mutagen-sensitive mutations (see Chapter 3) and P element transposons, whose transposition via a “cut and paste” mechanism induces a DSB at the site of excision, is being exploited in Drosophila melanogaster for the purpose of analyzing DSB repair in a multicellular organism. The full-length or “complete” P element is 2907 bp in length and has 31-bp inverted repeat termini (1) (Fig. 1). Functionally, it is a single gene consisting of four exons, numbered 0–3. In the germline, these are spliced together to produce an ∼2.5-kb transcript specifying an 87-kDa transposase required for transposition (2,3). In somatic cells, a 97-kDa host-encoded protein binds at a site near the 3′-end of exon 2, preventing its splicing to exon 3 (4,5). This alternative transcript specifies a truncated polypeptide of 66-kDa that acts as a repressor of P element mobility (6). An in vitro-modified P element, Δ2–3, in which the intron between exons 2 and 3 has been precisely deleted, expresses a transposase that is active in all tissues (2). Several stably integrated derivatives of Δ2–3 have been isolated, which express high levels of transposase without undergoing mobilization themselves (7).