Functional classification of 1-1 orthologs with a conserved expression context (score higher than zero). From all species pairs, all 1-1 orthologs that could be assigned to a KOG were included. The categories are grouped in the four main KOG classes. The horizontal dashed lines are the fraction of genes with a conserved expression context for the entire class. The functional categories are (the number between brackets is the number of genes with a conserved expression context): "Cellular processes and signaling" (D: Cell cycle control, cell division, chromosome partitioning (n = 442), M: Cell wall/membrane/envelope biogenesis (n = 73), N: Cell motility (n = 23), O: Posttranslational modification, protein turnover, chaperones (n = 1330), T: Signal transduction mechanisms (n = 1151), U: Intracellular trafficking, secretion, and vesicular transport (n = 953), V: Defense mechanisms (n = 67), W: Extracellular structures (n = 111), Y: Nuclear structure (n = 96), and Z: Cytoskeleton (n = 378)), "Information storage and processing" (A: RNA processing and modification (n = 823), B: Chromatin structure and dynamics (n = 244), J: Translation, ribosomal structure and biogenesis (n = 1153), K: Transcription (n = 985), and L: Replication, recombination and repair (n = 545)), "Metabolism" (C: Energy production and conversion (n = 486), E: Amino acid transport and metabolism (n = 367), F: Nucleotide transport and metabolism (n = 205), G: Carbohydrate transport and metabolism (n = 452), H: Coenzyme transport and metabolism (n = 131), I: Lipid transport and metabolism (n = 383), P: Inorganic ion transport and metabolism (n = 228), and Q: Secondary metabolites biosynthesis, transport and catabolism (n = 71)) and "Poorly characterized" (R: General function prediction only (n = 1716), S: Function unknown (n = 912), and X: Not categorized by NCBI staff (n = 2)) .