To analyze gene expression in Drosophila leg discs where an eye field was ectopically induced ([dppblink-GAL4, UAS-GAL4/UAS-ey] third instar wandering larvae), one to two hundred leg imaginal discs were dissected and immediately transferred into 800ul Trizol (GIBCO Life Technologies) and stored at –70°C until RNA extraction, which was performed according to the manufacturer’s instructions. Quantity and quality of total RNA were determined by capillary electrophoresis on a RNA6000 bioanalyzer (Agilent Technologies). Biotinylated RNA targets were prepared from 15 to 20 ug total RNA (500-700 leg discs) according to the standard Affymetrix procedure (see Michaut et al., 2003).