Abstract We have used an allele-specific oligonucleotide strategy to follow the activity of the Drosophila melanogaster Formosa Sgs-7 gene introduced by P element transformation into an Oregon-like strain in which the Sgs-7 allele differs by a single base in the coding sequence. We detect abundant activity of the inserted gene in a construct previously studied for the activity of the related Sgs-3 gene and conclude that the specific developmental regulation of Sgs-7 requires no more than 400 bp of 5′ flanking sequence. We also show more general applications of oligonucleotide probes for studying the tissue and developmental distribution of transcripts, for screening for transcripts in related species and show their convenience for the rapid synthesis of probes.