Some of the enriched categories point to particular pathways with large numbers of PSGs. A striking example is the complement immunity system, a biochemical cascade responsible for the elimination of pathogens. This system consists of several small proteins found in the blood that cooperate to kill target cells by disrupting their plasma membranes. Of 28 genes associated with this pathway in KEGG [33] , nine are identified as PSGs (FDR,0.05), and five others have nominal P,0.05 ( Figure S1 ). Most of these PSGs are inhibitors (DAF, CFH, CFI) and receptors (C5AR1, CR2), but some are part of the membrane attack complex (C7, C9, C8A), which punctures cell membranes to initiate cell lysis. Many of these PSGs are known to interact with one another, suggesting possible co-evolution. Two of three biochemical pathways known to activate the complement system are also enriched for PSGs (the classical complement pathway [P = 6.1610 27 ] and the alternative complement pathway [P = 1.5610 26 ]), as is the coagulation cascade that interacts with the complement system (''blood clotting,'' MWU P = 2.2610 27 ; Table S3 ). Other pathways that contain multiple interacting PSGs include those for apoptosis, taste transduction, antigen processing and presentation, and cytokine-and chemokine-mediated signaling (e.g., Figures. S4, S5) .