sentences:  Mice that are homozygous for this modified gene are called KIKI (knock-in knock-in), and the compound heterozygotes formed by crossing KIKI mice with frataxin knockout mice are called KIKO (knock-in knock-out).However, even KIKO mice still express 25-36% of the normal frataxin level, and show very mild symptoms.The final approach involves creating transgenic mice with a GAA-expanded version of the human frataxin gene.