The absence of KDELR2 cannot be compensated by the other KDEL receptors and leads to recessive OI type XXI, characterized by reduced intracellular levels of HSP47, which in turn causes a decrease in intracellular FKBP65, thereby overlapping with OI types X and XI [163]. The gene discussed is FKBP10; the disease is osteogenesis imperfecta.