BMC Genomics 2009, 10(Suppl 1):S14 http://www.biomedcentral.com/1471-2164/10/S1/S14 FCiogmurpear3ison of duplication age distributions between different groups of genes Comparison of duplication age distributions between different groups of genes. The x-axis was the evolutionary age (from now) in million years. The y-axis was the density of the distributions. A) GPCRs vs. the "downstream genes"; B) GPCRs vs. G Proteins; C) Between different subclasses of GPCRs (The densities was multiplied by the number of genes in correspond- ent subclasses to reflect the differences in frequencies); D) GPCRs with peptide-ligands vs. non-peptide ligands. In general, our results have shown that more duplication events were found for the GPCRs than the "downstream genes", particularly in more recent evolutionary stages when the duplication events were mostly local. This was consistent with the current theory of gene duplication and gene network. However, we have found several aspects that have not been covered in the current theory. First, at the time of emergence of early vertebrate, both GPCRs and the "downstream genes" families experienced explosive expansion. The asymmetric duplication-divergence proc- ess may be a good model for gene duplication at normal times, but more factors were likely to be in play at that Page 5 of 10 (page number not for citation purposes)