BMC Genomics 2009, 10(Suppl 1):S14 http://www.biomedcentral.com/1471-2164/10/S1/S14 was just the linear interpolation based on the distances of the event in the linearized tree to the species-split times. In cases that no bracketing species-split times can be found for a duplication event (usually for very ancient or recent duplication events), linear extrapolation was used to calculate the age. Expression profiles of human genes by UniGene We used the EST counts in UniGene [20] as the expression profiles of the human genes in the tissues. The ratio of the number of EST clones of a gene vs. the total number of clone in the library in a tissue was treated as the expression level of the gene in the tissues. For each gene, we calcu- lated its mean expression level and the standard deviation across the 45 tissues covered in UniGene. The distribution of the expression levels of a gene across species generally fit a gamma distribution with small mean and large varia- tion. We defined a gene highly expressed in a tissue if its expression level was one standard deviation greater than the mean expression level. Competing interests The authors declare that they have no competing interests. Authors' contributions XG, ZS, YZ and YH conceived the study. XG coordinated the project and the manuscript preparation. YH, YZ and ZS carried out the data analysis and drafted the manu- script. XG and ZS coordinated the project and the manu- script preparation. All authors read and approved the final manuscript. Additional material Additional file 1 Duplication age distribution of GPCR genes. Click here for file [http://www.biomedcentral.com/content/supplementary/1471- 2164-10-S1-S14-S1.xls] Additional file 2 Duplication age distribution of "downstream genes". Click here for file [http://www.biomedcentral.com/content/supplementary/1471- 2164-10-S1-S14-S2.xls] Acknowledgements This work was partially supported by grants from Iowa State University (to X.G.), a grant from National Natural Science Foundation of China (30700140 to Z.S.), and supported by Shanghai Leading Academic Discipline Project, Project Number: B111. This article has been published as part of BMC Genomics Volume 10 Supple- ment 1, 2009: The 2008 International Conference on Bioinformatics & Computational Biology (BIOCOMP'08). The full contents of the supple- ment are available online at http://www.biomedcentral.com/1471-2164/ 10?issue=S1. References 1. Donoghue PC, Purnell MA: Genome duplication, extinction and vertebrate evolution. Trends Ecol Evol 2005, 20(6):312-319. 2. Gu X, Wang Y, Gu J: Age distribution of human gene families shows significant roles of both large- and small-scale duplica- tions in vertebrate evolution. Nat Genet 2002, 31(2):205-209. 3. 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