Temperature sensitive mutants are a type of conditional mutants. They only grow at permissive temperatures and not at restrictive temperatures. i.e, They do not grow at higher temperatures as the proteins dont fold properly at this temperature and hence do not function. In most model organisms, identification of alleles responsible for this phenotype is challenging , nevertheless years of random mutageneis have identified a few alleles that are mutated for generating temperature sensitive mutants. An example is illustrated in the following paper linked here.