The opioid receptor types are nearly 70% identical, with the differences located at the N and C termini.
An IUPHAR subcommittee[REF][REF] has recommended that appropriate terminology for the 3 classical (μ, δ, κ) receptors, and the non-classical (nociceptin) receptor, should be MOP ("Mu OPiate receptor"), DOP, KOP and NOP respectively.