The mouse receptor tyrosine kinase (RTK) NEP, also called Ptk-3, is widely expressed, with high levels in proliferating neuroepithelia of mouse embryos. The recently described human discoidin domain receptor (DDR) has a predicted amino acid sequence 93% identical to that of murine NEP and may be its human homologue. We have mapped the gene encoding NEP in human and mouse by fluorescence in situ hybridization using a mouse cDNA probe. The NEP/Nep gene maps to human chromosome 6p21.3 and mouse chromosome 17C, respectively. This places the NEP/Nep gene at, or near, the major histocompatibility (MHC) locus--HLA in human and H2 in mouse, respectively. Based on its pattern of expression during development, NEP and Nep represent candidate genes for several MHC-linked developmental abnormalities in human and mouse.