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