Bohlin et al. BMC Genomics 2010, 11:464 http://www.biomedcentral.com/1471-2164/11/464 oxygen requirement, t, t = 1,2,3, are the effects of growth temperature, la, a = 1,2, are the effects of pathogenicity and hh, h = 1,...4, are the effects of habitat. b and g are the regression coefficients for the continu- ous variables GC and Size, respectively. Based on the inference using the regression model described by (3) we eliminated the non-significant vari- ables and obtained a reduced set of predictors: GC, phy- lum and oxygen requirement. In this reduced model, we included phyla only as an interaction with GC. The rea- son for this is that genomes within the same phylum tend to have similar GC content. Hence, a main effect of phylum may actually be a phylum-dependent GC effect. The model formulated as follows: E(GCVARoy ) =  +  o + ( +  y ) ⋅ GC (4) The ay in this model are defined as regression coeffi- cients for each of the 22 phylum categories. To test for possible associations between aerobiosis and mean genomic GC content, a regression model was fitted with GC as the response variable and aerobiosis as a group variable: E(GC) =  +  y +  o (5) μ, ay, δo are the same effects as those described for Equation (3). Acknowledgements The authors thank Colleen Ussery for critically reading the manuscript and the reviewers for their insightful suggestions. Author details 1Norwegian School of Veterinary Science, Department of Food Safety and Infection Biology, Ullevålsveien 72, P.O. Box 8146 Dep, NO-0033 Oslo, Norway. 2National Veterinary Institute, Section of epidemiology, Ullevålsveien 68, Pb 750 Sentrum, N-0106 Oslo, Norway. 3Norwegian University of Life Sciences, Department of Chemistry, Biotechnology and Food Sciences, Ås, Norway. 4University of Oslo, Department of Informatics, Pb. 1080, 0316 Oslo, Norway. 5Center for Biological Sequence Analysis, Department of Systems Biology, Comparative Genomics Unit, Technical University of Denmark, DK- 2800 Lyngby, Denmark. 6Centre for Molecular Biology and Neuroscience (CMBN), Institute of Medical Microbiology, Rikshospitalet, NO-0027 Oslo, Norway. Authors’ contributions JB wrote the paper and carried out analyses. LS, ABK, JB, ES carried out statistical analyses. TD suggested the study. DWU, SPH, ES, JB, KL performed biological analyses. TD, KL, ES, SPH and DWU critically drafted and revised the manuscript. All authors have read and approved the final manuscript. 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