A syndrome including deficient linear, endochondral, and radial bone growth associated with severe cervico-thoracic lordosis, decreased intrathoracic volume, atelectasis, and early death has been noted in mice possessing the phenotypes of the recessive mutant genes pallid (pa) and wellhaarig (we) as the result of recombination of chromosome 2 between these genes. The syndrome is not seen in the parental strains, which are homozygous for the chromosomal segment containing one or the other gene (pa +/pa + or + we/+ we), nor in the intercross mice heterozygous for both genes in the trans configuration (pa+/+we). The abnormal offspring appeared randomly in the breeding colony with no F1 breeding pair producing more than one pa we mouse. These observations rule out the presence of a mutant gene, fixed or unfixed, as an explanation for this syndrome. We hypothesize that the syndrome is the result of the complementary action of the genes or the chromosomal segments containing the genes pa and we or weBkr. The posited synergism is further supported by the finding that we, which functions as a recessive gene in mice of the pa/+ genotype, appears to function as a dominant gene in mice possessing the pa/pa genotype.