Over the past decade, a number of techniques have been developed that allow the introduction of defined, cloned DNA sequences into
animal germ lines. Once inserted, these sequences, now called trans-
genes, are stably passed on from generation to generation. In other
words, the transgene becomes a part of the genetic make-up of that
particular line of animal. Every indivIdual of a particular line will
carry the transgene in every cell of its body. Of fundamental impor-
tance has been the observation that transgenes are often expressed-
that is they are functional-and that this expression is subject to correct
tissue-specific, developmental, and physiological regulation. It is
therefore now possible to analyze the role and regulation of specific
cloned genes WIthin the whole organism. Such organisms are called
transgenic organisms.