A construct containing three tandem stop codons in exon 3b and a neomycin resistance gene was inserted to interrupt transcription of the smooth muscle isoform. Exons 4 and 5 were deleted upon recombination. Lung, bladder, intestine and uterus of mutants showed the lack of smooth muscle isoform in Western blot analyses. The non-muscle isoform was translated at normal levels. (J:97432)