sentences:  In 1976 Martin reported that the effects of N-allylnormetazocine (SKF-10,047) could not be due to activity at the μ and κ receptors (named from the first letter of their selective ligands morphine and ketazocine, respectively) and a new type of opioid receptor was proposed; σ (from the first letter of SKF-10,047).The opioid classification was eventually dropped however resulting from it not possessing the canonical opioid G-protein coupled receptor structure and the receptor was later referred to as simply the σ1 receptor.It was found to have affinity for the (+)-stereoisomers of several benzomorphans ("e.g.