Genes predicted to be involved in the metabolism of saturated fatty acids are more expanded than those involved in the metabolism of unsaturated fatty acids. Using a compendium of microarray expression experiments (Methods), we compiled a list of genes upregulated in the presence of different fatty acid sources. We found that genes upregulated under unsaturated conditions have more uniform phylogenetic profiles, while those upregulated under saturated conditions, cholesterol or ceramide have expanded through duplications in pathogenic Mycobacteria ( Figure 4 ). Saturated fatty acids and cholesterol are more prevalent in an animal host than in the soil, which contains mostly unsaturated fatty acid from plant inputs. Since it is believed that Mtb uses cholesterol as a carbon source within the host [32] , this could reflect an Figure 2 Summary of SYNERGY results: Number of gains, losses, and duplication at each node. For each node, the node number is marked in black; the total number of genes present at each node is indicated in red, and the numbers of gains, losses, and duplications are indicated in parenthesis in blue http://www.broadinstitute.org/ftp/pub/seq/msc/pub/SYNERGY/index.html.