Normal tissues are characterized by a balance between cellular stasis, cell proliferation, cell differentiation and cell death. Aberrant regulation of any of these cell processes can result in cancer. Cell death during embryogenesis, tissue atrophy and normal tissue turnover is called apoptosis. This is characterized by cytoplasmic and nuclear condensation, nuclear disorganization and fragmentation of genomic DNA into 180-200 base pair oligomers. Several human cDNA fragments have been shown to render MCF-7 cells resistant to cell death induced by Pseudomonas exotoxin, Pseudomonas exotoxin-derived immunotoxins, diptheria toxin and tumor necrosis factor (TNF). One such fragment has proven to be the human homolog to the yeast chromosome segregation homolog, CSE1. Cloning of the full-length human cDNA has revealed a putative protein designated CAS, for cellular apoptosis susceptibility, that is 971 amino acids in length with 59% overall sequence homology as compared to yeast CSE1. CAS is highly expressed in testis and fetal liver.