Neutrophil Elastase (NE) is a 220 residue, single chain glycoprotein that functions as a potent serine protease. NE can degrade Omp, a structural protein localized on the cell wall of Gram-negative bacteria, and also has the capacity to attenuate the pathogenicity of invading microbes by targeting their virulence factor. NE is expressed in developing granulocytes in the bone marrow, stored in the azurophilic (primary) granules of mature neutrophils and released upon neutrophil activation or disintegration. Despite the fact that neutrophils carry large amount of NE, and are capable of synthesizing a variety of proteins, the NE gene is not expressed in mature neutrophils. Also, NE can be used as an additional marker to differentiate the involvement of neutrophilic myeloid cells. This marker is useful for differentiating leukemic infiltrates of myeloproliferative processes in lymph nodes and other organs from undifferentiated carcinomas and/or histiocytic sarcomas or large cell lymphomas.