Hi all, that maybe a stupid question but here it is anyway: I m currently analyzing microarray data coming from different platforms (single and 2-color arrays) and technologies and I observe a non random distribution of high intensity spots over the slides I m analyzing (after and before normalization). In the case of 2 color microarray differentially expressed genes are not randomly distributed over the slide and it looks like waves over the slides for most of these. Is it something normal ? I dont see a clear gradien like washing problems would give... thanks for the input Philippe