Genome stability is maintained by a network of proteins that ensure faithful DNA replication and efficient response to DNA damage. Variation in levels of proteins across the cell cycle, between tissues and even through natural fluctuations are common 
and could influence genome stability especially for proteins that are present in limiting amounts. Proteins with limited expression are likely to be weak links in genome maintenance and, therefore, could be risk factors in disease, especially cancer predisposition, when combined with environmental stress. This could be particularly important for the cases where small, environmentally relevant amounts of genotoxins inhibit a mutation avoidance repair system 
. Even a cell with WT genotype may be at risk for genome instability due to fluctuation in expression of limiting proteins.
Many genes are involved in spontaneous and damage-induced homologous recombination (HR) ensuring efficiency and accuracy. The repair of double-strand breaks (DSBs) by HR is an evolutionarily conserved process (for review, see 
) and is generally considered error free since it uses information from an undamaged DNA template. However, since HR can also occur between related as well as identical sequences it can lead to genomic instability through loss-of-heterozygosity (LOH) and nonallelic recombination between repeats across the genome, which can result in chromosome rearrangements 
. These changes are often detected in genetic disorders, cancer and during evolution (discussed in, 
Mutations in HR components can lead to genome instability and cancer predisposition 
. Increased genome instability can also result from changes in the amounts of wild type gene products functioning in HR. In yeast, a genome wide analysis identified 178 genes with haplo-insufficiency causing increased chromosome loss in the heterozygote state 
. Included was RAD55
, which is directly related to HR; it showed both chromosomal instability and sensitivity to DNA damage when heterozygous. Haplo-insufficiency for several human genes leads to DNA damage sensitivity, genome instability and/or cancer susceptibility, suggesting they are present in amounts that are limiting for HR 
We sought to identify more proteins that are present in limiting amounts for HR-mediated DSB repair and to assess the consequences of reduced levels. The identification of proteins that when limiting affect genome stability can be accomplished through manipulation of gene dosage in polyploid cells. Small variations in the amount of a protein can be accomplished with tetraploid strains of the budding yeast Saccharomyces cerevisiae
where gene dosage can be varied over a factor of 4 from one (simplex) to four copies (tetraplex; referred to as WT) by deleting copies of the gene from homologous chromosomes. This scheme provides the opportunity to address the relationship between gene dosage and biological consequences for many genes. It also enables studies reduced amounts of essential gene products. Importantly, unlike other systems for down-regulating proteins, the amount of a protein can be reduced without affecting the coding sequence or other transcription/translation controls of the remaining alleles. This approach was used for the yeast photolyase DNA repair gene PHR1 
which can reverse UV-induced pyrimidine dimers and the RAD52
which is essential for recombinational repair of DSBs 
We applied the reduced gene dosage approach to three genes that impact HR: RAD50
, and MCD1
. The MRX complex in yeast, which includes Rad50, is responsible for DSB recognition and DNA resection, the first step in HR and in DNA damage signaling at site-specific and in damage-induced genome wide DSBs (
and references therein). The Rad51 protein which is directly involved in recombination including homology search and formation of joint molecule (for review see, 
) was previously suggested to be present in limiting amounts 
. We found that changes in levels of Rad50 and Rad51 did not affect the response to ionizing radiation.
We also investigated the consequences to genome stability of reducing the dosage of genes affecting sister chromatid cohesion. While not directly involved enzymatically in HR 
, the sister chromatid cohesion complex (cohesin) that includes Mcd1, Smc3, Smc1 and Irr1 is important in DSB repair in haploid yeast cells (
and for review, see 
). Following induction of DSBs, cohesin is recruited to DSBs via the DNA damage response pathway 
. The cohesin becomes cohesive even at undamaged sites of the genome 
. Although cohesin facilitates DSB repair between sister chromatids, its impact when homologous chromosomes are present is unknown. Recombination between sister chromatids is generally acknowledged to be more efficient than between homologous chromosomes 
suggesting that cohesin inhibits recombination between homologous chromosomes. In this sense, cohesin might suppress opportunities for LOH as well as nonallelic recombination and chromosome rearrangements involving repeated DNAs. Previously it was shown that cohesin can influence the pattern of recombination induced by a single DSB in a plasmid based assay 
. However, since cohesin is an essential gene and viable mutants are likely to be sensitive to ionizing radiation it is not known what role it might play in maintaining recombination fidelity when survival is high.
Here we show that even a modest reduction in the level of cohesin dramatically increases the ability of γ–radiation to induce recombination between homologous chromosomes in the G2 but not the G1 phase of the cell cycle even at low radiation doses when survival is high. This finding, which also extends to UV-induced recombination, suggests that cohesin confines recombinational repair to sister chromatids even in the absence of DSBs, thereby reducing the risk of genome instability.