The present disclosure provides methods and compositions related to the modification of immune effector cells to increase therapeutic efficacy. In some embodiments, immune effector cells modified to reduce expression of one or more endogenous target genes, or to reduce one or more functions of an endogenous protein to enhance effector functions of the immune cells are provided. In some embodiments, immune effector cells further modified by introduction of transgenes conferring antigen specificity, such as exogenous T cell receptors (TCRs) or chimeric antigen receptors (CARs) are provided. Methods of treating a cell proliferative disorder, such as a cancer, using the modified immune effector cells described herein are also provided.