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). RNA extraction, 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).