The PSGs are enriched for a wide variety of functions related to immunity and defense. Several over-represented categories describe activation in response to external or environmental stresses, such as from bacteria (P = 4.2610 28 ), viruses (P = 3.0610 28 ), wounding (P = 3.2610 28 ), and acute inflammation (P = 4.7610 211 ). In some cases, different categories reflect the same or very similar sets of genes (e.g., ''response to wounding'' and ''acute inflammatory response,'' or ''response to virus'' and ''response to bacterium''), while in others they reflect quite distinct gene sets (''response to wounding'' and ''response to virus'') ( Figure 2 ). Genes involved in both innate (P = 1.9610 29 ) and adaptive (P = 1.5610 25 ) immunity are over-represented, with many PSGs contributing to both classes. The conventional division of adaptive immunity into humoral (P = 1.6610 27 ) and cellular (P = 3.5610 27 ) responses is reflected in the enriched GO categories. Various mechanisms of immune response are represented, including previously identified categories for natural killer cell (P = 1.6610 28 ), B-cell (P = 4.8610 27 ), and T-cell (P = 1.2610 28 ) mediated immunity [5, 8] , and new categories such as cytokine/chemokine-mediated (7.6610 28 ) and complement-mediated immunity (P = 6.0610 26 ; see Table S3 ).