More than 10 years ago, the technique was established for the production of transgenic animals by the introduction of foreign genes into the genome of mammals. 1 – 4 Most of the first experiments were done in mice (reviewed in References 5 to 13), but in the meantime the methodology has been extended to other vertebrate species like rats, 14 – 16 rabbits, 17 – 20 sheep, 17, 21 – 25 goats, 26 cattle, 23 – 25, 27 pigs, 17, 20, 24, 25, 28 – 30 fish, 31, 32 and birds. 33 Transgenic domestic animals have been developed in order to increase the quality and the yield of their economically exploited products. 19, 25, 28 – 30, 32 – 34 In addition, it is possible to express transgenes in the mammary gland of these animals, leading to the secretion of pharmaceutically interesting proteins in considerable amounts in the milk. 22 – 24, 26, 27, 34 In order to produce transgenic animals other than mice, the peculiarities of the physiology of the other species and the distinct physical properties of their oocytes had to be taken into account.