A neomycin resistance gene was inserted into exon 5, which encodes amino acids 109 through 154. Northern blot analysis of RNA isolated from colon, lung, and aorta tissue showed the presence of a shortened transcript in homozygous mutant mice. Sequence analysis of the mutant message identified a cryptic donor site in exon 5 that led to the excision of 84 nucleotides encoding amino acids 127 through 154. Transfection of microsomes, showed that the mutant protein product was incapable of calcium-uptake.