The obtention of the first transgenic animals in 1980-1982 was a logical fo
llowing to the works aiming at isolating genes and transferring them into c
ultured cells to study their mechanism and their role in thecontrol of biol
ogical functions. The transfer of a gene into an animal corresponds its rei
ntroduction in the complex environment of a whole living organism. Although
transgenesis still suffers from technical imperfections, it is an invaluab
le and unique tool for biologists and biotechnologists who develop projects
in medicine and agronomy. The main problems of transgenesis are the relati
vely low yield of transgenic generation and the ill-controled expression of
the transgenes. Moreover, transgenesis has intrinsic limits which are due
to the complexity of the living organisms. Introducing a foreign gene in e
genome or inactivating an endogenous gene consists in adding or subtracting
an information in the exceedingly complex network formed by the multiple i
nteractions between the different genes and gene products. Despite the limi
tation, transgenesis is more and more used by experimenters. The complete s
equencing of several genomes, including human genome, urges to implement tr
ansgenesis systematically to define the function of the newly discovered an
d unknown genes.