A deletion of ~6 kb within intron B was identified in the 129X1/SvJ background. The deleted region included 24 bp of the exon 2 splice acceptor site. RT-PCR analysis identified transcript in which exon 2 is skipped and exon 1 splices directly to exon 3. The aberrant splicing generates a frameshift and ultimately a premature stop codon at position 2 of exon 3. Neither normal protein nor truncated fragments were detected by Western blot analysis. This mutation has been detected in 129S1/SvImJ, 129S2/SvPas, 129S4/SvJae, 129P2/OlaHsd, FVB/N, NZB/BlNJ, and NZW/LacJ backgrounds but not in C3H or C57BL/6J backgrounds.