An IRES-lacZ-neomycin resistance cassette replaced the most of the second exon including almost the entire DNA binding bHLH region and most of the coding region, and placed lacZ under control of endogenous transcriptional elements. RT-PCR analysis of brain did not detect transcript in homozygous mutant E18.5 embryos. Beta-galactosidase was expresssed under the control of the endogenous promoter. (J:73967)