The mouse eosinophil peroxidase (mEPO) gene was cloned by screening a random-primed bone marrow cDNA library at reduced criteria using a hEPO cDNA, An mEPO cDNA was subsequently used to isolate the mEPO gene from a h-genomic library, The mEPO gene displays a high degree of conservation with its human homologue: the transcription units are approximately the same size, conserve the relative size and position of the 12 exons associated with each gene, and at a nucleotide level the mouse and human EPO genes are 86% identical in the protein coding regions and 66% identical in the 3'-untranslated trailer regions, This strong conservation extends to the encoded proteins which show similar to 90% amino acid identity, Expression of the mEPO gene is restricted to tissues containing eosinophil progenitor cells (e,g,, bone marrow and spleen), a pattern similar to the expression of another murine eosinophil granule protein, major basic protein.