title: Monozygotic twins:  identical in name only.
authors.forename: Ray
authors.initials: R
authors.lastname: Greek
authors.forename: Mark J
authors.initials: MJ
authors.lastname: Rice
mesh_terms: D015760:Alfentanil / Q000494:pharmacology*; D000701:Analgesics, Opioid / Q000494:pharmacology*; D005260:Female; D006801:Humans; D008297:Male; D012054:Reinforcement, Psychology
chemical_list: D000701:Analgesics, Opioid; D015760:Alfentanil