Oxysterol-binding proteins, or Oxysterol-binding protein related-proteins, or simply ORPs, are evolutionary related proteins involved in sterol synthesis and/or its regulation. These include mammalian oxysterol-binding protein (OSBP), a protein of about 800 amino-acid residues that binds oxysterols (oxygenated derivatives of cholesterol), cholesterol and phosphatidylinositol 4-phosphate (PI4P); yeast Osh1, a protein of 859 residues that plays a role in ergosterol synthesis; and yeast proteins HES1 and KES1, highly related proteins of 434 residues that seem to play a role in ergosterol synthesis
^Jiang, B.; Brown, J. L.; Sheraton, J.; Fortin, N.; Bussey, H. (1994). "A new family of yeast genes implicated in ergosterol synthesis is related to the human oxysterol binding protein". Yeast10 (3): 341–353. doi:10.1002/yea.320100307. PMID8017104.