“…Generally, for such analyses, stable transgenic animals have been produced in Caenorhabditis elegans (Fire 1986), sea urchins , Drosophila (Rubin and Spradling 1982), Xenopus (Etkin et al 1984), mouse (Palmiter and Brinster 1986;Jaenisch 1988), and recently in fish (for a review see Maclean et al 1987;Stuart et al 1988;Guyomard et al 1989). However, stable transgenic systems have several limitations: (1) production of stable transgenic lines is time con,suming and therefore only a limited nurober of regulatory sequences can be tested; (2) integration site and integrated copy nurober can obscure the true characteristics of such sequences; and (3) genomic imprinting, as reported in the mouse (Surani et al 1988), may additionally alter the expression of the transgene.…”