An 8.5-kb genomic region extending from -3300 to +8200 with respect to the major transcription initiation site was replaced with a neomycin selection cassette. This deletion encompasses the minimal promoter, exon 1 and intron 1. RNase protection assays revealed that mRNA accumulation in various heterozygous tissue samples (liver, spleen, muscle, heart, testis, and brain) is about one half of that in wild-type samples.