Nuclear matrix proteins (NMPs) make up the internal structural framework of the nucleus and are associated with such functions as DNA replication, RNA synthesis, and hormone receptor binding. Work by Nakayasu and Berezney, Zeitlin et al., and Berrios et al. has indicated that NMPs are involved in regulation and coordination of gene expression. The identification of cell-type specific nuclear matrix proteins supports their potential contribution to cellular differentiation and tissue development. Although the nuclear matrix has been shown to be highly insoluble in vitro, it is now known that cell death releases soluble nuclear matrix proteins that can be detected in culture supernatant and other fluids containing dead and dying cells. The antibodies employed in the Cell Death Detection (Nuclear Matrix Protein) ELISA were developed to nuclear matrix proteins isolated from tissue culture cells by standard procedures. Both antibodies have been shown to react with lambda gt11 cDNA clones carrying published sequence from nuclear mitotic apparatus protein (NuMA), a nuclear matrix protein. More specifically, the Capture Antibody for NMP 41/7 reacts with a lambda gt11 cDNA clone encoding a peptide of approximately 60 kDa, located in the amino terminus of the published sequence for NuMA, and the FITC-anti-NMP 41/7 detector antibody reacts with a lambda gt11 cDNA clone encoding a peptide of ~96 kDa, located in the carboxy-terminus of the published sequence for NuMA. Because the level of NMP 41/7 detected in culture supernatant is a function of the number of dead and dying cells, the assay may be useful to quantify cell death. Other methods used to measure cell death include MTT, 51CR, trypan blue staining and ATP level. In comparison to these methods, the NMP 41/7 assay is advantageous since it measures antigen released directly from dying cells, therefore allowing differentiation between cytostatic and cytotoxic effects. Also, the NMP 41/7 assay requires only a small aliquot of material for testing, and does not require manual cell counts for quantification. In addition to measuring cell death in tissue culture, NMP 41/7 has been shown to be a useful marker for quantitative determination of engraftment, extension of disease and treatment response of human leukemia in SCID mice. The level of NMP 41/7 detected in SCID mouse serum correlates with human-derived tumor mass because the Cell Death Detection (Nuclear Matrix Protein) ELISA antibodies detect only human nuclear matrix protein. The antibodies have been shown to be reactive with a number of human cell lines; they may not react with all human cells. The antibodies have not been shown to be reactive with non-human cells. The user should make an initial determination of the assay's suitability for a particular use.