RESCUE OF THE EN-1 MUTANT PHENOTYPE BY REPLACEMENT OF EN-1 WITH EN-2

The related mouse Engrailed genes En-1 and En-2 are expressed from the one- and approximately five-somite stages, respectively, in a similar presumptive mid-hindbrain domain. However, mutations in En-1 and En-2 produce different phenotypes. En-1 mutant mice die at birth with a la rge mid-hindbrain deletion, whereas En-2 mutants are viable, with cere bellar defects. To determine whether these contrasting phenotypes refl ect differences in temporal expression or biochemical activity of the En proteins, En-1 coding sequences were replaced with En-2 sequences b y gene targeting. This rescued all En-1 mutant defects, demonstrating that the difference between En-1 and En-2 stems from their divergent e xpression patterns.