The gene was disrupted using a two-step targeting strategy. First, exons 2-10 were replaced with a neo-TK cassette via homologous recombination. Next, correctly targeted ES cells were subjected to another round of gene targeting to replace the neo-TK cassette with a lacZ cassette. Successful gene replacement was selected for via gancyclovir resistance. Homozygous mutant animals were successfully established. (J:79629)