Relationships between the genome, proteome, diseaseome and drugome. The number of distinct protein species (about 400,000) comprising the proteome (green circle, scaled down by 25% relative to the other circles), is estimated by taking the approximately 25,000 currently annotated genes (yellow circle) and assuming about four splice variants per gene and about four post-translationally modified proteins per splice variant. The genome, diseaseome and drugome form a Venn diagram. The red circle represents the approximately 1,800 genes known to be involved in various diseases (the diseaseome). Of these, a small fraction (the drugome) is targeted by FDA-approved drugs. Not all drug targets have been characterized as disease genes. In total, proteins encoded by approximately 400 genes (0.1% of the proteome) are targeted by about 1,200 FDA-approved drugs. There are more drugs than protein targets because more than one drug can target the same protein.