A portion of exon 4 and all of exon 5 were replaced with a lacZ-neo cassette inserted by homologous recombination. Northern blot and RT-PCR analysis identified two transcripts produced by the mutant allele. A shorter transcript encoded a protein in which the amino terminal of the endogenous protein was adjoined to lacZ. A longer transcript was the result of the aberrant splicing of exons 3 and 6. Protein produced from the longer transcript was detected by Western blot analysis and lacked a central region that included the carboxy terminal half of the second zinc finger (aa 505 to 2889).