Fragment containing a ligand-regulated destabilizing domain derived by mutagenesis from the Homo sapiens FKBP1A gene (HGNC:3711). 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, but this degradation can be blocked by the addition of the cell-permeable, synthetic 'Shield1' ligand (PMID:16959577).