Myeloperoxidase (MPO; EC 1.11.1.7) is a lysosomal hemeprotein that plays an important role in the host defense mechanism against microbial diseases. This neutrophil disorder, characterized by the lack of MPO, may result in a weakened defense activity. Complete MPO deficiency has been postulated to be to originate from genomic mutation. Recently, two Japanese patients were reported with MPO deficiency. Both had base substitutions in the exon 9 region of the MPO gene; a region in close proximity functionally important residue, His502. Genomic DNA from 387 Japanese individuals was examined to determine the prevalence of these recently discovered base substitutions. None of these DNA samples possessed the mutations found in the MPO deficient cases, though two synonymous and one non-synonymous mutation were found. The frequency of mutation in the exon 9 coding region was estimated to be one heterozygote in 129, thus the homozygote of such mutations would be revealed one in 16,000 in the Japanese population.