To isolate transcripts that are differentially expressed at early stages of Drosophila development, performed a SSH (suppression substractive hybridization) procedure between gastrulation and syncytial blastoderm stages, using cDNA prepared from stage 6-7 embryos (gastrula) as tester and that from stage 2-3 embryos (syncytial blastoderm) as driver, thus generating a cDNA population enriched in cDNA fragments that were expressed at a higher level in gastrula.