A neomycin selection cassette replaced a region spanning the start codon in exon 1 and 5.8kb of intronic sequence following exon 4. In addtion to the initial methionine, the deleted sequence encoded the Sin3-interacting domain, a conserved HGYAS motif, and the basic and helix-loop-helix regions of the bHLH leucine zipper. RT-PCR analysis and in situ hybridization confirmed the absence of transcript in homozygous mutant embryos.