Double-helix substance (3)
I believe the answer is:
dna
(Other definitions for dna that I've seen before include "Briefly, substance in chromosomes used by police to identify criminals" , "Substance in chromosomes that stores genetic data" , "Bodily substance that stores genetic information - police use it to identify" , "Genetic carrier" , "Deoxyribonucleic acid (abbreviation)" .)