RAE-1delta is one of the five RAE-1 family, GPI-linked membrane protein consisting of alpha, beta, gamma, delta, and epsilon. They are strong homology within the family, related by 92%-95% sequence identity. They are distantly related to MHC class I proteins. RAE-1 proteins are abundantly expressed in fetal tissues, but not in normal adult tissue. They are constitutively expressed on some tumors and can be induced by retinoid acid. Its ligand NKG2D is found on NK cells, many transformed cell lines, and LPS-stimulated peritoneal macrophages. The interaction of NKG2D with RAE-1 transmits stimulatory signals to NK cells and other hematopoietic cells, leading to enhanced proliferation, cytokine secretion and target killing. They are involved in the regulation of innate and immune cytotoxic responses to tumors, pathogen-infected cells, and autoimmune diseases.