A stable genomic source of P element transposase in Drosophila melanogaster is described. The P element, called P[ry⁺Δ2-3](99B), is highly active in mobilizing other elements but remains remarkably stable. It was constructed by F. Laski, D. Rio, and G. Rubin and has been found to be useful for experiments involving P elements. This element can be used for P element mutagenesis and efficiently substitute for helper plasmids in P element-mediated transformation. It can also be used to move transformed elements around the genome. The P element family of transposons is involved in the syndrome of genetic effects known as P-M hybrid dysgenesis. These effects include male recombination, mutation generation, chromosomal rearrangements, and germline destruction at high temperatures, leading to gonadal dysgenic sterility. These effects result from the mobilization of P elements when they are released from regulation by P cytotype, a repressive condition characteristic of P strains. Complete P elements are 2.9 kb in length with inverted terminal repeats required for transposition. Four open reading frames code for an 87-kD transposase polypeptide. Restriction of P element activity to the germline results from differential splicing of the mRNA such that the third (2-3) splice occurs only in the germline. The nature of P genotype remains unclear, although it may be related to failure of this splice and the resultant production of a truncated 66-kD polypeptide. P elements have been developed as an important tool in genetics and molecular biology in Drosophila melanogaster. They are used for insertional mutagenesis, primarily for transposon tagging of genes for cloning. However, their transposable nature and multiple copies cause problems. Novel insertion mutations must usually be stabilized in the P genotype, where numerous other inserts complicate cloning. P elements are also used as transformation vectors. Because transformed genes are inserted at various locations in the genome, their expression is subject to position effects. To compensate, it is useful to collect many inserts. This has been accomplished by large scale transformation and by the subsequent mobilization of inserts by reinjection of a "helper" element. A more efficient alternative is to use a genomic transposase source. For example, Cooley, Kelley, and Spradling (1988) have used an element they call "Jumpstarter" for this purpose, and others have used a progenitor of Jumpstarter. Ideally such an element should not be mobile itself. In this report, a modified P element that is particularly capable of mobilizing other elements, yet is itself remarkably stable, is described. This element, called Δ2-3(99B), is characterized in detail. It has high levels of transposase activity and is remarkably stable. It can be used to improve the efficiency ofA stable genomic source of P element transposase in Drosophila melanogaster is described. The P element, called P[ry⁺Δ2-3](99B), is highly active in mobilizing other elements but remains remarkably stable. It was constructed by F. Laski, D. Rio, and G. Rubin and has been found to be useful for experiments involving P elements. This element can be used for P element mutagenesis and efficiently substitute for helper plasmids in P element-mediated transformation. It can also be used to move transformed elements around the genome. The P element family of transposons is involved in the syndrome of genetic effects known as P-M hybrid dysgenesis. These effects include male recombination, mutation generation, chromosomal rearrangements, and germline destruction at high temperatures, leading to gonadal dysgenic sterility. These effects result from the mobilization of P elements when they are released from regulation by P cytotype, a repressive condition characteristic of P strains. Complete P elements are 2.9 kb in length with inverted terminal repeats required for transposition. Four open reading frames code for an 87-kD transposase polypeptide. Restriction of P element activity to the germline results from differential splicing of the mRNA such that the third (2-3) splice occurs only in the germline. The nature of P genotype remains unclear, although it may be related to failure of this splice and the resultant production of a truncated 66-kD polypeptide. P elements have been developed as an important tool in genetics and molecular biology in Drosophila melanogaster. They are used for insertional mutagenesis, primarily for transposon tagging of genes for cloning. However, their transposable nature and multiple copies cause problems. Novel insertion mutations must usually be stabilized in the P genotype, where numerous other inserts complicate cloning. P elements are also used as transformation vectors. Because transformed genes are inserted at various locations in the genome, their expression is subject to position effects. To compensate, it is useful to collect many inserts. This has been accomplished by large scale transformation and by the subsequent mobilization of inserts by reinjection of a "helper" element. A more efficient alternative is to use a genomic transposase source. For example, Cooley, Kelley, and Spradling (1988) have used an element they call "Jumpstarter" for this purpose, and others have used a progenitor of Jumpstarter. Ideally such an element should not be mobile itself. In this report, a modified P element that is particularly capable of mobilizing other elements, yet is itself remarkably stable, is described. This element, called Δ2-3(99B), is characterized in detail. It has high levels of transposase activity and is remarkably stable. It can be used to improve the efficiency of