Gene targeting is a powerful approach of studying the gene function in vivo. Specific genetic modifications, including simple gene disruption, point mutations, large chromosomal deletions and rearrangements, targeted incorporation of foreign genes, could be introduced into the mouse genome by gene targeting. Recent studies make it possible to do the gene targeting with temporal and spatial control. Gene targeting is a powerful approach of studying the gene function in vivo. Specific genetic modifications, including simple gene disruption, point mutations, large chromosomal deletions and rearrangements, targeted incorporation of foreign genes, could be introduced into the mouse genome by gene targeting. Recent studies make it possible to do the gene targeting with temporal and spatial control.