A targeting vector containing a 755-756 CA to GG substitution resulted in a Ser252Trp mutation in exon IIIa. A floxed neo was inserted into intron IIIa (exon 7). This allele was a hypomorph and either was not expressed or expressed in reduced levels. Expression of alternative transcripts IIIb and IIIc were reduced by 16% to 41% compared to normal transcripts in multiple tissues tested.