1) They create changes 
Chemical agents that increase the rate of genetic mutation by interfering with the function of nucleic acids. A clastogen is a specific mutagen that causes breaks in chromosomes. ... (12 Dec 1998) ... 
Agents that cause an increase in the rate of mutation; includes X-rays, ultraviolet irradiation (260nm), and various chemicals.
are those substances that can alter (mutate) the genes - the bits of material that pass on characters in living cells.