This family includes eukaryotic, prokaryotic and archaeal proteins that bear similarity to a C-terminal region of human activator of 90 kDa heat shock protein ATPase homolog 1 (AHSA1/p38). This protein is known to interact with the middle domain of Hsp90, and stimulate its ATPase activity. It is probably a general upregulator of Hsp90 function, particularly contributing to its efficiency in conditions of increased stress. p38 is also known to interact with the cytoplasmic domain of the VSV G protein, and may thus be involved in protein transport. It has also been reported as being underexpressed in Down's syndrome. This region is found repeated in two members of this family.