Abstract: Method for identifying compounds that interfere with or modulate sister chromatid separation in animal or plant cells by modulating a protease with separin-like cysteine endopeptidase activity. Inhibitors of separin activity are useful in cancer therapy, to prevent birth defects and to increase ploidy in plants.

Abstract: Compounds which interfere with the function of the cdc6 protein to form or maintain pre-replication complexes in an animal cell without impairing the cell's ability to activate cyclin dependent kinases that promote S phase and/or M phase for use in cancer therapy due to their ability to kill tumor cells by inhibiting DNA replication. Method for screening for such compounds.

Abstract: A eukaryotic expression vector includes a controllable repressor operator sequence witin the expression control sequence of the vector. The controllable repressor operator sequence represses expression of a heterologous structural gene located downstream of the expression control sequence in the presence of a gene product or a combination of gene products of a yeast mating type locus. This allows control of the expression level of the heterologous structural gene. Controllable repressor operator sequences responsive to the a1/.alpha.2 and .alpha.2 gene products of the MATa and MAT.alpha. yeast mating type locus alleles are described, both derived from genetic material and synthesized chemically.