What are transgenic mice and what do we learn from them? In this revie
w, we focus on the generation of ''classical'' transgenic and ''knock-
out'' mice. The establishment of transgenic and gene-targeted mice pro
vides an unique tool to study the function(s) of a given gene in the c
ontext of a whole organism. Eased on selected examples, we demonstrate
the potential of this transgenic technology to understand the interac
tions between cells, organs and organ systems in genetically engineere
d mice.