Schematic diagram representing the evolution of proeukaryotic (A) and eukaryotic (B) HSP90 family of genes. The filled bars indicate gene duplication events. A) We propose that HSP90-like genes in Bacteria and Archaea are basal to HTPG genes that only exist in Bacteria. HTPG genes underwent two gene duplication events that gave rise to three groups (Group A, B and C). B) Eukaryotic members were derived from HTPG Group A, and evolved into four subfamilies TRAP (mitochondrial), HSP90A (cytosolic), HSP90B (ER) and HSP90C (chloroplast) throughout three gene duplication events. One additional gene duplication event led to the division of HSP90A into HSP90AA and HSP90AB in vertebrates. The organismal groups in parenthesis indicate where these HSP90 gene copies are found.