We have identified the gene responsible for the inherited zinc deficiency in the lethal milk (lm) mouse. The gene, here designated Znt4, encodes a 430-amino-acid protein that is homologous to two proteins, ZnT2 and ZnT3, responsible for sequestration of zinc into endosomal/lysosomal compartments and synaptic vesicles, respectively. We show that the Znt4 gene confers zinc resistance to a zinc-sensitive yeast strain and that it is abundantly expressed in the mammary epithelia and brain. The lethal milk mutant has a nonsense mutation at arginine codon 297 in the Znt4 gene.