Overall distribution of orthologs among reference genes. Most of the reference genes (1,088) had only one ortholog on each of the U95A, U74A and U34A arrays used in these studies. The first bar shown here represents the number of reference genes that had three orthologs. The majority of remaining reference genes had two orthologs on one of the studied arrays. Overall, about 62% of reference genes had at least one multiple ortholog set.