A recent article by Galitski and Roth((1)) characterizes adaptive reve
rsion of chromosomal lac(-) mutations in Salmonella typhimurium LT2. U
sing a classical genetic approach they show that adaptive reversion, a
s characterized by the appearance of late revertant colonies, is an ex
ception rather than a general phenomenon for reversion of nonsense, mi
ssense, frameshift and insertion mutations. For certain mutations, how
ever, the number of late revertants exceeds the predicted number. Thes
e excess revertants suggest that adaptive mutability is applicable to
chromosomal genes as well as to genetic changes involving F plasmids a
nd lysogenic phages.