The "code" in Walter Isaacson's best-selling 2021 book "The Code Breaker" (3)
I believe the answer is:
dna
(Other definitions for dna that I've seen before include "It carries genetic information" , "Substance that stores genetic information in body" , "Double-helix molecule (abbr.)" , "Genetic carrier" , "Briefly, substance in chromosomes used by police to identify criminals" .)