Transgenic animals, mostly mice, are of great value in the genetic analysis of inherited diseases and cancer. Here we are interested not so much in adding a cloned transgene as in discovering the function of genes already present. The general approach is to inactivate, or “knock out,” the gene of interest and then ask what defect this causes. To achieve this the target gene is first cloned. Then the gene is disrupted by inserting a DNA cassette into its coding sequence. (Most DNA cassettes also include an antibiotic resistance gene for easy detection.) The intruding DNA segment prevents the gene from making the correct protein product and thus abolishes its function. The inactive copy of the gene is then put back into the animal by following the procedure outlined earlier for transgene insertion. The incoming DNA, carrying the disrupted gene, sometimes replaces the original, functional, copy of the gene by homologous recombination. Founder mice are obtained that have one copy of the disrupted gene. By breeding them together, mice with both copies disrupted are obtained. Such mice are known as knockout mice and will completely lack gene function ( Fig. 15.4 ). If the gene in question is essential, homozygous knockout mice may not survive, or may live only a short time.