A RES file (.res) is a tab-delimited text file that contains gene expression data. The GCT and RES files are the same, except that the RES file format contains labels for each gene's absent (A) versus present (P) calls as generated by Affymetrix's GeneChip software. The RES file format is described on the GenePattern web site: See GCT File Format for a discussion of how IGV determines the loci for the gene expression data.