A spontaneous mutation generated a duplication of exons 28 and 29 and frameshift mutation (AAC to TGA) after exon 29. RT-PCR confirmed the reduction in transcript expression in bone marrow cells and splenocytes. This duplication is present in C57BL/6NHsd but not C57BL/6NClr, C57BL/6NJ or C57BL/6NTac.