isolation of a tight conditional mutant of a gene of interest is an effective way of studying the functions of essential genes strategies that use ubiquitin mediated protein degradation to eliminate the product of a gene of interest such as heat inducible degron td and auxin inducible degron aid are powerful methods for constructing conditional mutants however these methods do not work with some genes here we describe an improved aid system iaid for isolating tight conditional mutants in the budding yeast saccharomyces cerevisiae in this method transcriptional repression by the tet off promoter is combined with proteolytic elimination of the target protein by the aid system to provide examples we describe the construction of tight mutants of the replication factors dpb11 and mcm10 dpb11 iaid and mcm10 iaid because dpb11 and mcm10 are required for the initiation of dna replication their tight mutants are unable to enter s phase this is the case for dpb11 iaid and mcm10 iaid cells after the addition of tetracycline and auxin both the tet off promoter and the aid system have been shown to work in model eukaryotes other than budding yeast therefore the iaid system is not only useful in budding yeast but also can be applied to other model systems to isolate tight conditional mutants.