Temperature-sensitive mutant

The permissive temperature is the temperature at which a temperature sensitive mutant gene product takes on a normal, functional phenotype. When a temperature sensitive mutant is grown in a permissive condition, the mutated gene product behaves normally (meaning that the phenotype isn't observed), even if there is a mutant allele present. This results in the survival of the cell or organism, as if it were a wild type strain. In contrast, the nonpermissive temperature or restrictive temperature is the temperature at which the mutant phenotype is observed.

Most temperature sensitive mutations affect proteins, and are recessive and cause loss of protein function at the non-permissive temperature. The permissive temperature is one at which the protein typically can fold properly, or remain properly folded.