Pax3^Sp-1H, splotch-Harwell 1. This mutation arose after paternal acute X-irradiation with spermatogonial sampling. It was found as a cluster of 2 mutations and behaves like the original Pax3^Sp mutation. The heterozygote shows ventral white spotting, often extending to the feet and tail. Lethal Pax3^Sp-1H homozygotes and Pax3^Sp-1H/Pax3^Sp heterozygotes produce rachischisis and other abnormalities (J:14096). Studies of Pax3^Sp-1H homozygotes using a lacZ transgene show a gradual size reduction of spinal and sympathetic ganglia along a rostrocaudal gradient as well as a total absence of sympathetic ganglion cells in the thoracic and lumbar regions (J:4077). Comparative studies of Pax3^Sp-d homozygous, Pax3^Sp-1H homozygous, and Pax3^Sp-d/Pax3^Sp-1H heterozygous embryos show that failure of the neural crest-derived septum of the truncus arteriosis is directly proportional to the number of Pax3^Sp-1H alleles; aortic conus malformations are observed in all the embryos (J:14376). Cardiac neural crest migration is also abnormal in Pax3^Sp-2H mice (J:38385). In rare cases, neural tube and tail defects can occur in offspring of Pax3^Sp-1H/+ matings without the appearance of neural crest defects (J:3467). Motor innervation of the hind limbs develops normally in Pax3^Sp-1H mutant mice, despite the lack of Schwann cells accompanying the spinal nerve outgrowth (J:1310). Pax3 and its mutants affect peripheral nervous system myelination, presumably through the effect on Schwann cells (J:28908).