Cover picture: The cover image illustrates the use of principal components analysis to show variability between 162 Affymetrix GeneChips. This method was used to investigate variation in global gene expression profiles during development of three regions of the rat brain. Each sphere represents a single microarray, with ellipsoids drawn to encompass variability within each biological group. The top graph uses different colors and ellipsoids to identify different developmental stages from embryonic day 16 (left) to postnatal day 90 (right). Below, the same graph is redrawn to identify the three brain regions investigated. For details, see the article by Stead et al. in this issue (pages 345–353).

(pages 345–353)