To eliminate the translation of isoform B, the start codon of isoform B was mutated to encode leucine (ATG to CTG). A single loxP site remained in an intron after a floxed selection cassette was excised via cre mediated recombination. While isoform B was undetected by Western blot analysis of homozygous mutant uterine extracts, the presence of isoform A was confirmed.