Genes encoding opioid receptors (including opioid-like receptor (ORL) proteins) can be retrieved from vertebrate libraries using the murine probe disclosed herein under low-stringency conditions. The DNA sequence shown in FIG. 5 or its complement can be used to obtain the human delta, kappa and mu genes as well as the murine mu gene and human ORL-1. The probe provided encodes the murine delta opioid receptor.