Total RNA was quantified using RiboGreen and split into equal aliquots of approximately 10 ng, representing RNA from approximately 10,000 cells, and labeled using a total of three rounds of RNA amplification, exactly as described previously (Scherer et al. <a href="http://www.ncbi.nlm.nih.gov:80/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12661160&dopt=Abstract" target="_blank" class="normal">2003</a>). Labeled cRNA was fractionated and hybridized to the U74Av2 microarray following standard Affymetrix protocols.