Quantitative gene expression analysis using RNA array. RNA samples from controls and drug treated rats were arrayed on a glass slide in two blocks of 100 spots consisting of 10 different samples (columns), each with 10 replicate spots (rows) and hybridized with fluorescently labeled probes (A and B). After hybridization and washing, the slide was scanned on a microarray scanner, resulting in two pseudo-color images for Cy3-labeled Atpi and Acoa1 (A) and Cy5-labeled 18S ribosomal RNA for normalization (B). A smaller array format with a block of 40 spots consisting of 10 different samples (columns), each with 4 replicate spots (rows) is shown in (C). Shown here (Serpina1) is one of 5 transcripts analyzed in parallel hybridizations on separate slots of a single slide. For comparison, Northern blot hybridization images for the Atpi, Acoa1, Serpina1 and 18SrRNA probes are shown in (D).