Tryptophan-deficient mutant is used for the reverse mutation, while galactose utility or resistance to 5-methyltryptophan may be used for forward mutation.

Resistance to 8-azaguanine in S. typhimurium - Similar to Ames test, but instead of reverse mutation, it checks for forward mutation that confer resistance to 8-Azaguanine in a histidine revertant strain.

A role for 'amelioration', whereby structural characteristics of the foreign gene are eventually homogenized to resemble those of the recipient species, has been assumed, but the rate was postulated to be the same as the rate of random, forward mutation [ 42 ] .