Hypoxia up-regulated protein 1, also known as 15 kDa oxygen-regulated protein, 17 kDa glucose-regulated protein, ORP-15, GRP-17, and HYOU1, is a member of the heat shock protein 7 family. Seven members from four different heat shock protein (HSP) families were identified including HYOU1 (ORP15), HSPC1 (HSP86), HSPA5 (Bip), HSPD1 (HSP6), and several isoforms of the two testis-specific HSP7 chaperones HSPA2 and HSPA1L. HYOU1 is highly expressed in tissues that contain well-developed endoplasmic reticulum and synthesize large amounts of secretory proteins. It is highly expressed in the liver and pancreas. HYOU1 is also expressed in macrophages within aortic atherosclerotic plaques and in breast cancers. HYOU1 has a pivotal role in cytoprotective cellular mechanisms triggered by oxygen deprivation. It may play a role as a molecular chaperone and participate in protein folding. Suppression of HYOU1 is associated with accelerated apoptosis. It is suggested to have an important cytoprotective role in hypoxia-induced cellular perturbation. This protein is up-regulated in tumors, especially in breast tumors, and thus it is associated with tumor invasiveness.