Three human mRNAs are regulated post-transcriptionally by iron via iron-responsive elements (IREs) contained in each mRNA. A cytoplasmic protein (IRE-BP) binds to these cis-acting elements and mediates the translational regulation of ferritin H- and L-chain mRNA and the iron-dependent stability of transferrin receptor (TfR) mRNA. We have taken advantage of the different mobilities of the human and rodent IRE/IRE-BP complexes on non-denaturing polyacrylamide gels to determine the chromosomal localization of the gene encoding the IRE-BP. Utilizing a panel of 34 different human/rodent hybrid cell lines we have assigned the IRE-BP gene to human chromosome 9. This new technique based on nucleic acid/protein interaction may allow determination of the chromosomal localization of other RNA- or DNA-binding proteins.