Abstract: A method for genetically modifying a target population of an organism, comprising the steps of 1. providing a modified organism, wherein the modified organism is capable of sexually reproducing with an organism of the target population, and wherein a selected gene in the germline of the modified organism is disrupted by having inserted into it a sequence-specific nonMendelian selfish gene; 2. introducing the modified organism into the target population. A method for genetically modifying a target population of an organism, comprising the steps of 1. Introducing a homing endonuclease gene (HEG) into the germline of an organism which is capable of sexually reproducing with organisms of the target population; 2. introducing the modified organism into the target population. The HEG may encode a recombinant endonuclease, for example an endonuclease with a zinc-finger DNA binding domain and a sequence non-specific nuclease domain.