No coding sequence alterations were found, but a single G-to-A base pair change was identified in the first base of intron 4 that eliminates the exon 4 splice donor site by changing it from G-GT to G-AT. RT-PCR from samples derived from E10.5 embryos showed that in mutant homozygotes a 44 base pair extension of exon 4 into intron 4, was present that is predicted to result in an in-frame stop codon located immediately after exon 4 to be read and to terminate the translation of the encoded after 223 amino acids plus one altered amino acid. QRT-PCR confirmed reduced transcript expression at E10.5.