Mice homozygous for the spontaneous mutation tkb can have one or more than one kink or bend in the tail, but this mutation has very low penetrance. Heterozygous intercrosses generated only 4.2% mutants instead of the 25% expected, and intercrosses of homozygotes and heterozygotes generated only 8.1% mutants instead of the 50% expected. Auditory brainstem response analysis and ophthalmoscopic examination found no defects in the hearing or eyes. This mutation was also found to have very low penetrance in mapping crosses to FVB/NJ, CAST/EiJ, C3H/HeSnJ, and 129S1/SvImJ. The underlying genetic defect has not been mapped or sequenced.