A targeting construct was designed to replace exons 2 and 3 with a neomycin resistance cassette, thereby preventing expression of the 71-kDa protein, but allowing expression of the 35-kDa protein. Northern and immunoblot analyses confirmed absence of wild type transcript and protein in mutant. A truncated 0.5 kb mRNA was detected in mutant mice, a truncated protein corresponding to the long isoform was not detected. (J:107006)