A synthetic oligodeoxynucleotide complementary to endorphin mRNA and a method of using it to detect and isolate endorphin mRNA and cDNA from human and rabbit pancreas. A unique 15 base oligodeoxynucleotide dCATGAACCCGCCGTA wherein T represents thymine, G represents guanine, A represents adenine, and C represents cytosine and where at least 13 of the 15 nucleotides are as indicated (one of those indicated may be instead one of the other three mentioned nucleotides) has been found to be complementary to endorphin mRNA. To isolate endorphin mRNA, total RNA is first extracted from human brain and A+ is isolated from the total RNA. Other tissues may be used. The A+ RNA is then treated with the oligodeoxynucleotide, and the resulting hybridized RNA is enzymatically converted to endorphin mRNA:cDNA which can then be purified and used in a conventional manner to produce endorphin by cloning techniques.