The replication of cells and successful maintenance of cell lineages depend on at least two things. First, the cell must successfully replicate its genetic material and faithfully pass an intact copy of the genome to its progeny at cell division. Second, at least one of the two resulting cells must survive to carry the genome on to the next generation. Tumor cells are particularly adroit at the process of cell replication, out-performing their normal counterparts at the expense of the host organism. However, cancer cells are also known for their genetic instability, and aneuploidy can result when chromosome segregation occurs erroneously. Fortunately, checkpoints that link apoptosis to defects in cell division provide some measure of protection, but dysregulation of apoptosis can create a fertile soil in which genetically unstable cells can thrive.

A family of proteins containing a zinc binding fold, termed the BIR domain, has recently been implicated in chromosome segregation and cytokinesis. Some members of this family also regulate apoptosis. The mammalian prototype of this family is Survivin, an∼ 16 kDa protein that contains a BIR domain, followed by a long α-helical region important for its targeting to the mitotic-spindle, spindle midbody, and related structures (