AbstractHomozygous Splotch mutant mice (Sp/Sp) die on day 14 of gestation with neural tube defects, curly tail, and malformations of neural crest derivatives. Sp1Hmice, which have a radiation‐induced allele of Splotch with a similar phenotype, were used for this study. The neural tube defects are always located in the lumbosacral region and in 50% of the cases also in the region of the hindbrain. In this report, rare cases of neural tube defects and tail defects among the offspring of crosses between Splotch (Sp1H) heterozygotes are presented, which are not associated with a neural crest defect. This suggests that the development of the neural tube and neural crest defects in this mutant is caused by independent mechanisms or is dependent on the dosage of the mutant gene, with different thresholds being pathogenetic in the neural tube and neural crest, respectively. © 1992 Wiley‐Liss,