Sister-chromatid cohesion, the process of pairing replicated chromosomes during 
mitosis and meiosis, is mediated through the essential cohesin complex and a 
number of nonessential cohesion genes, but the specific roles of these 
nonessential genes in sister-chromatid cohesion remain to be clarified. We 
analyzed sister-chromatid cohesion in double mutants of mrc1Delta, tof1Delta, 
and csm3Delta and identified additive cohesion defects that indicated the 
existence of at least two pathways that contribute to sister-chromatid cohesion. 
To understand the relationship of other nonessential cohesion genes with respect 
to these two pathways, pairwise combinations of deletion and 
temperature-sensitive alleles were tested for cohesion defects. These data 
defined two cohesion pathways, one containing CSM3, TOF1, CTF4, and CHL1, and 
the second containing MRC1, CTF18, CTF8, and DCC1. Furthermore, we found that 
the nonessential genes are not important for the maintenance of cohesion at 
G(2)/M. Thus, our data suggest that nonessential cohesion genes make critical 
redundant contributions to the establishment of sister-chromatid cohesion and 
define two cohesion pathways, thereby establishing a framework for understanding 
the role of nonessential genes in sister-chromatid cohesion.