The  Ames  test  exposes  auxotrophic
   strains  of  Salmonella  (his^)  to  a chemical compound to assess its
   mutagenicity.

   Liver   extract  is  added,  since  metabolic  modification  of  these
   compounds in the liver may make them mutagenic.

   The  mutagenicity  of  a  chemical  is  reflected  in the frequency of
   reverse mutations, which yields wild-type (his^+) prototrophs.