Experimental data used for populating the ortholog-link database. (a) Using Affymetrix MAS 5.0 software, absent (black), marginal (white) and present (gray) transcript-abundance calls were counted for each experimental dataset and the values obtained expressed as a percentage of all calls. (b) By masking poorly performing probes for U95A, U74A and U34A, the present call ratio for these GeneChips was increased by 25%. As dog mRNA was hybridized to the human U133A chip, the present call ratio for this hetero-hybridization was much lower than that in other experiments. We therefore corrected U133A probe sets for differences in gene sequence between human and canine, which increased the present call ratio by more than 50%.