Mice homozygous for the blunt tail mutation have a tail that is slightly shorter than normal with a blunted, not tapered, tip and irregular regions of reduced pigment on the tail. This phenotype can be detected by two to three weeks of age. Homozygotes have normal fertility and lifespan.
The blunt tail (blnt) mutation arose spontaneously in a C57BL/6J strain congenic for a BALB/cJ-derived interval on Chromosome 9. This mutant subline has been maintained via backcross-intercross to C57BL/6J so the BALB/cJ-derived sequence has either been bred out or diminished. In 2013 this strain reached generation N5 and sperm was cryopreserved from homozygous males.