Exon 23 was replaced with modified sequence in which nucleotide susbtitutions within 3 polypyrimidine tracts and an AG dinucleotide were made in order to disrupt an internal splice acceptor site specific to the beta isoform. Transcript encoding the beta isoform was undetected by RT-PCR analysis of homozygous mutant hepatic mRNA, while wild-type levels of transcript encoding the alpha isoform were observed. Analysis of MEFs reiterated these findings at the protein level.