422 bp of the 5' untranslated region and the first 144 bp of the gene coding region were replaced with a PGK-neo cassette via homologous recombination. The gene targeting event results in deletion of the first 48 amino acids of the basic helix-loop-helix domain. Gene expression is absent in kidneys from homozygous mutant animals as determined by in situ hybridization. (J:76787)