A member of the cytochrome P450 superfamily has been identified within region 2D of the Drosophila X chromosome. The sequence of this gene shows strongest homology with P450 family 4 genes, and has thus been named CYP4D1 in accordance with convention. This P450 gene is expressed throughout Drosophila development, with the highest levels of transcript accumulation occurring during late larval stages. A fragment of Drosophila genomic DNA including the CYP4D1 gene has been reintroduced into the germ line by P-element-mediated transformation. This transduced fragment does not rescue any of the lethal mutations that have been identified in this genetically well-characterized region of the Drosophila genome.