Illumina, Inc. offers a whole-genome expression product for rat that analyzes twelve samples in parallel on a single Illumina BeadChip. Content for the new RatRef-12 Expression BeadChip is derived from the NCBI RefSeq database (Release 16), with over 22,000 rat transcripts represented. The multi-sample format makes it useful for studies in toxicology, biomarker discovery and disease modeling where many samples need to be processed with high uniformity.