Midkine (Mdk) and heparin-binding neurotrophic factor (Hbnf)/pleiotrophin (Ptn) comprise the Midkine family of developmentally regulated signaling molecules. We have determined the chromosomal localization of these genes in the mouse by use of single-strand conformation polymorphisms (SSCPs), which facilitated the typing of Mdk and Hbnf alleles in recombinant inbred (RI) strains and interspecific backcrosses. Mapping was performed relative to other cloned genes, as well as simple sequence length polymorphisms (SSLPs) in the interspecific backcrosses. Mdk maps to mouse Chromosome (Chr) 2, linked to the Hoxd gene cluster. Hbnf maps to proximal mouse Chr 6, linked to the Cftr and Cpa genes. Comparative mapping of human MDK and HBNF employing species-specific polymerase chain reaction (PCR) primers and human monochromosomal somatic cell hybrids assigns MDK to human Chr 11 and HBNF to human Chr 7q32-qter.