The spontaneous tpr mutation was identified at the University of Michigan. RT-PCR revealed that the sequences corresponding to exons 3 and 4 are absent in the encoded transcript. This results in a frame shift leading to premature termination at residue 91 in exon 5. PCR on genomic DNA confirmed that exons 3 and 4 are deleted in this mutant. The predicted truncated protein could not be detected immunologically. (J:84411)