Hi Jack and Shirley, There is another reference that could be worth looking into. This one is based on RNA-seq transcriptome data, as opposed to microarrays and ESTs. Here it is: Ramskold D, Wang ET, Burge CB, Sandberg R (2009) An Abundance of Ubiquitously Expressed Genes Revealed by Tissue Transcriptome Sequence Data. PLoS Comput Biol 5(12) Hope that helps. - Al. [[alternative HTML version deleted]]