As mentioned above, biologically central (BC) genes that we analyze include: aging, cancer, HIV, and pathogen-interacting genes. We obtained them from the following databases. Aging genes (A) are human genes implicated in the process of aging that are available from AnAge Databank -Human Ageing Genomic Resources (http://genomics.senescence.info/) [62] . Cancer genes (C) are human genes implicated in cancer that are available from: Cancer Gene Database (http://ncicb.nci.nih.gov/projects/cgdcp), Cancer Genome Project -the Cancer Gene Census (http://www. sanger.ac.uk/genetics/CGP/Census/) [63] , GeneCards (http:// www.genecards.org/) [64] , Kyoto Encyclopedia of Genes and Genomes (KEGG) (http://www.genome.jp/kegg/disease/) [65] , and Online Mendelian Inheritance in Man (OMIM) (http://www. ncbi.nlm.nih.gov/sites/entrez?db = omim) [66] . HIV genes (HIV) are human genes known to interact with genes of the HIV virus [63] that are available from HIV-1-Human Protein Interaction Database (http://www.ncbi.nlm.nih.gov/RefSeq/HIVInteractions/) [67] . Finally, pathogen-interacting genes (PI) are human genes known to interact with genes of pathogens [43] . The data are downloaded in 2009 and 2010.