Disclosed herein are compositions for inactivating the human TCR-alpha gene comprising engineered LAGLIDADG homing endonucleases (LHEs) and their derivatives, particularly derived from members of the \-Onul subfamily of LHEs. Polynucleotides encoding such endonucleases, vectors comprising said polynucleotides, cells comprising or having been treated with such endonucleases, and therapeutic compositions deriving therefore are also provided.