A 2 nucleotide insertion of GG is detected at the start of exon 2 in a stretch of consecutive G nucleotides (9 in the wild-type C57BL/6 sequence) resulting in a frameshift mutation that generates a premature termination codon. Western blot analysis with an antibody directed against amino acids 496-509 does not detect the protein band in homozygous embryonic stem cells. This mutation is found in the tw5 haplotype.