Transgenic research usually involves a large investment in carrying out a series of low-efficiency steps to produce a few transgenic founder animals, and then characterizing a transgenic line of animals derived from each founder. There is considerable variation in phenotype among transgenic lines made with the same DNA construct. In the case of livestock species, to produce one founder animal that expresses a transgene typically requires injection of DNA into hundreds of embryos, which then are transferred to reproductive tracts of recipients for gestation to term. To reduce costs, embryos sometimes are screened by analysis of a biopsy using the polymerase chain reaction, and only those with the transgene are transferred. Characterizing a transgenic line often is a greater logistical undertaking than making the transgenic founder. Ideally, animals should be evaluated for the transgenic trait as well as for absence of undesirable side effects in both sexes in both the hemizygous and homozygous transgenic states. Producing homozygous transgenic animals requires mating relatives, resulting in inbreeding. Characterization of transgenic lines takes many years in species with long generation intervals; the whole process is much less costly in litter-bearing species. Because of safety and efficacy issues, characterizing transgenic lines for agricultural production purposes will be more demanding than characterizing them for research purposes. At a minimum, three broad areas of expertise are required for successful transgenic projects: molecular biology, embryo micromanipulation/reproductive physiology, and knowledge of the biology of the tissues/systems affected by the transgene. Facilities and equipment attendant to these areas of expertise are essential, as are appropriate animal facilities, including surgical suites for most projects. Although current procedures are expensive and inefficient, new approaches plus incremental improvements that accumulate with experience will make transgenic technology much more efficacious in the future.
