p^un, pink-eyed unstable, recessive. This mutation arose spontaneously in the C57BL/6J strain. Homozygotes resemble p/p mice phenotypically, but pun reverts somatically to wild-type with high frequency. The reversion rate varies with age, being highest at 2 and at 10 days of gestation, and with genotype of parents, being higher in p^un homozygous progeny of heterozygous parents (p^un/+ or p^un/p mated inter se) than of homozygous pun parents (J:5526). Whitney and Lamoreux (J:6739) suggest that pun may have been produced by insertion of a transposable element into the wild-type p locus gene, frequent excision of the element being responsible for the high reversion rate. Subsequently Brilliant et al. (J:2230) have shown that in the pun mutation, coding sequences of the p gene are interrupted by a head-to-tail tandem duplication of genomic DNA, one copy of which appears to be lost in revertant mice (J:3762). In the pigmented retinal epithelium, darkly pigmented revertant cells occur with greater frequency in the more distal or anterior part of the epithelial layer than in the proximal part near the optic nerve, indicating that the rate of reversion is conditioned by the tissue environment (J:7305). Homozygous p^un mice, in common with some other mutations that reduce pigmentation, have a reduced number of projections of retinal ganglion cells to the ipsilateral lateral geniculate nucleus (J:6064).