undulated extensive mice carry a deletion of at least 28.2 kb, removing the terminal Pax-1 exon including the poly(A) signal. This mutation leads to a drastically reduced amount of Pax-1 transcript. The similarity in phenotype between unex and the point mutation un suggests both to be functionally equivalent. A third mutant, Undulated short tail, harbors a deletion of at least 48.3 kb, eliminating the Pax-1 locus as well as 38 kb of flanking sequences. Phenotypically, this mutant differs from un and unex. The comparative phenotype analysis of all undulated alleles revealed morphological alterations in the vertebral column, the sternum, the scapula, the facial skeleton, and the thymus that correspond to Pax-1 expression domains. The vertebral column phenotype is characterized by the reduction of vertebral components. These reductions can be traced back to the delayed cell accumulation in the embryonic sclerotome while the principal organization of this tissue is maintained. The morphological alterations outside the vertebral column are likewise confined to size reductions. Therefore, Pax-1 may exert similar functions in all of its expression domains.