HESAS (HERVs Expression and Structure Analysis System) database was developed to understand the human endogenous retroviruses (HERVs) that have an effect on the expression of human functional genes. The database products are generated by the exon-based expressed sequence tag clustering and reconstructing of partial HERV structures that result from various mutations during primate evolution. The expression types were classified according to the existence of splicing, transcriptional start and polyadenylation signal sites. The database currently contains HERV information on 26,981 human genes of exon-intron structure. The HERV elements were inserted into 17,317 of these genes and linked to expression with 898 genes.