We established a list of 2190 siRNAs where these phenotypes could be reliably estimated. This list can be seen as a resource to build new hypotheses on the associations between genes and biolog ical processes. However, due to the possibility of off target effects of siRNA perturbations, unavoidable experimental variability and the use of 17-AAG order a cell line with a heavily rear ranged genome, for general validity these results must be confirmed by independent assays, for instance, rescue experiments in another cell line. The spindle assembly checkpoint acts as a sur veillance mechanism by delaying the metaphase to ana phase transition until all the chromosomes have properly aligned and attached to the mitotic spindle, thus, preventing chromosome instability.

In the presence of even a single improperly attached kineto chore, SAC is activated to inhibit a large multisubunit E3 ubiquitin ligase complex, the anaphase promoting complex cyclosome, and prevents anaphase onset. APC C activity requires the association of Cdc20 in early mitosis, while Cdh1 is required to activate APC C in late mitosis and during G1. The primary target of SAC is the Cdc20 activator that, when inhibited, cannot activate APC C to degrade securin. Degradation of securin is required for activation of separase and cleavage of cohe sion between sister chromatids which in turn triggers anaphase onset in mitotic cells. The first identified components of SAC were isolated in two independent genetic screens in Saccharomyces cerevisiae and include MAD1, MAD2, MAD3, BUB1, and BUB3.

These proteins are widely conserved, both structurally and functionally, throughout the eukar yotic kingdoms. However, additional proteins essen tial for the checkpoint activity have continued to be discovered in higher eukaryotes. These include Rod, Zw10 and CENP F pro teins, among others. These components lack clear yeast orthologs, suggesting that, in higher eukaryotes, checkpoint signaling is more elaborate. The SAC components and the checkpoint signalling pathway are highly conserved in C. elegans. The C. ele gans homologues of the SAC components, originally dis covered in yeast, have been identified and named mdf 1, mdf 2, san 1, bub 1 and bub 3, respectively. Recent availability of knockout alleles of these checkpoint components, in addition to RNA interference experiments, allowed assessment of the phenotypic con sequences in the absence of the SAC gene products.

All of these genes are important for genome stability and viability in the presence of spindle damage. However, while mdf 2, san 1 and bub 3 become essential only in the presence of chemical or mutational disrup tions of the mitotic Cilengitide spindle, bub 1 and mdf 1 are essential for embryonic viability, long term survival and fertility under normal laboratory conditions in C. ele gans.