The floxed region containing exons encoding the DNA binding domain (exons 4 through 7) and a neo cassette was excised via the expression of cre recombinase in vivo. RT-PCR and sequence analyses showed the deletion to result in the aberrant splicing of exon 3 to exon 8 and generate a frameshift mutation. A premature stop codon was identified, precluding the translation of a protein product longer than 77 residues, resulting in a truncated protein similar to mutant proteins identified in patients with MHC-II deficiency.