Fragment containing a ligand-regulated destabilizing domain derived by mutagenesis from the Escherichia coli dihydrofolate reductase (ecDHFR) gene. When fused to a protein of interest, this destabilizing domain confers ligand-dependent stability on the tagged protein: in the absence of ligand, the tagged protein is constitutively and rapidly degraded by the proteasome, but this degradation can be blocked by the addition of trimethoprim (TMP), a cell-permeable ligand for the destabilizing domain (PMID:20851347).