Each record in GAD represents a specific gene from a unique publication of a human population based genetic association study and is categorized into one of 24 general disease classes corresponding to broad MeSH disease or disease phenotypic groupings. Table 1 is a summary of the number of positively associated human genes in each MeSH human disease class. As represented by these disease classes the GAD database covers a broad selection of diseases falling into major disease classes including; aging studies, cancer, immune disorders, psychiatric diseases, metabolic conditions, pharmacogenomic studies, and studies of chemical dependency, among others. Similarly, each record in the phenotype files from the MGI phenotype database represents a unique mouse gene specific genetic model. Table 2 shows the general categories represented by the mouse phenotype summary files and the number of mouse genes found in each top level phenotype class. The mouse files contain a greater number of intermediate developmental and morphological phenotypes (e.g. insulin resistance, absent CD4+ T cells, abnormal spatial learning) while the human files tend to comprise a greater number of end stage clinical disease phenotypes (e.g. Type 2 Diabetes, multiple sclerosis, autism). Table 3 introduces examples of human genes from fundamental biological pathways that have been consistently associated with major disease phenotypes highlighting the sometimes-broad pleiotropic effects that major regulatory molecules have on multiple disease phenotypes. Genes such as NOS3, nitric oxide synthase 3, regulating nitrous oxide production; HLA-DQB1, the MHC class II molecule DQ beta 1, involved in antigen presentation; ACE, angiotensin I converting enzyme, central to the renin-angiotensin system and PPARG, peroxisome proliferator-activated receptor gamma, regulating transcription in pathways important in lipid metabolism are examples of genes that affect multiple tissues and different organ systems through the complex course of disease progression. Importantly, all the mouse orthologs of the human genes in Table 3 have experimentally determined phenotypes that are similar or broadly overlapping with human clinical disease phenotypes (see below).