Previous studies have shown that En-1, a homeobox-containing gene, maps close to or at the Dh locus in the mouse. Since homeobox-containing genes are key genes in the control of development the close proximity of En-1 to the developmentally significant gene Dh raised the possibility that the Dh mutation represented a mutant allele of En-1. A genetic analysis involving En-1, Dh, and other chromosome 1 markers (Emv-17, In and Pep-3) shows that although Dh and En-1 are closely linked they are separable by recombination (4/563). The likely gene order and recombination frequencies of these loci are: In (5.2±0.9) Emv-17 (1.1±0.4) Dh (0.7±0.4) En-1 (3.0±0.7) Pep-3. This shows that Dh is not a mutant allele of En-1.