Abstract: Ligand-dependent inteins allow for modulation of a protein's activity in vivo. Upon binding of the ligand to the ligand-dependent intein inserted into the protein of interest, the hybrid protein undergoes protein splicing removing the intein. The activity of the spliced protein is then restored. A 4-hydroxytamoxifen-dependent intein based on the M. tuberculosis RecA intein is prepared and demonstrated in a variety of exteins contexts. The invention provides a system for engineering other ligand-dependent inteins and using them, including the ligand-dependent inteins themselves, hybrid proteins with the inserted ligand-dependent inteins, polynucleotides encoding inteins and hybrid proteins, and engineered cells. Kits with the materials and reagents necessary for preparing and using ligand-dependent inteins are also included.