Fragile sites are loci of recurrent chromosome breakage in the genome. They are 
found in organisms ranging from bacteria to humans and are implicated in genome 
instability, evolution, and cancer. In budding yeast, inactivation of Mec1, a 
homolog of mammalian ATR, leads to chromosome breakage at fragile sites referred 
to as replication slow zones (RSZs). RSZs are proposed to be homologous to 
mammalian common fragile sites (CFSs) whose stability is regulated by ATR. 
Perturbation during S phase, leading to elevated levels of stalled replication 
forks, is necessary but not sufficient for chromosome breakage at RSZs or CFSs. 
To address the nature of additional event(s) required for the break formation, 
we examined involvement of the currently known or implicated mechanisms of 
endogenous chromosome breakage, including errors in replication fork restart, 
premature mitotic chromosome condensation, spindle tension, anaphase, and 
cytokinesis. Results revealed that chromosome breakage at RSZs is independent of 
the RAD52 epistasis group genes and of TOP3, SGS1, SRS2, MMS4, or MUS81, 
indicating that homologous recombination and other recombination-related 
processes associated with replication fork restart are unlikely to be involved. 
We also found spindle force, anaphase, or cytokinesis to be dispensable. RSZ 
breakage, however, required genes encoding condensin subunits (YCG1, YSC4) and 
topoisomerase II (TOP2). We propose that chromosome break formation at RSZs 
following Mec1 inactivation, a model for mammalian fragile site breakage, is 
mediated by internal chromosomal stress generated during mitotic chromosome 
condensation.