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The proenkephalin precursor proteins belong to the opioid neuropeptide precursor family and are proteolytically processed to form active opiod peptides which function as ligands for opiod receptors. OGFR (Opioid growth factor receptor) is a 677 amino acid protein that localizes to both the nucleus and the cytoplasm and functions as a receptor for opiod growth factors. Expressed at high levels in liver and heart and at moderate levels in kidney, brain, pancreas and skeletal muscle, OGFR exists as multiple alternatively spliced isoforms and, via its ability to bind opiod ligands, is thought to play an important role in growth regulation, possibly functioning as a tumor suppressor. Upon DNA damage, OGFR may be phosphorylated by ATM or ATR. The gene encoding OGFR maps to human chromosome 20, which houses over 600 genes and comprises nearly 2% of the human genome.