Accurate DNA synthesis and repair of DNA lesions assure low rates of mutation. Genetic defects in either lead to increased, genome-wide mutator phenotypes which can manifest in cancer predisposition 
. Strong mutators can cause reduced fitness due to the accumulation of dysfunctional alleles which place them under continual negative selection pressure 
. By contrast, there would be little selection against transient hypermutability within subpopulations of cells or within limited genomic regions 
. High mutation frequencies during transient hypermutability might contribute to overall population mutability and could represent a prominent source of multiple mutations important for evolution and cancer. Transient increases in localized mutation can even be regulated and benefit an organism, as found for somatic hypermutability in the immunoglobulin genes where a coordinated system of proteins induce DNA damage and repair the lesions in an error prone manner 
Examples of unregulated transient hypermutability include multiple mutations 
, adaptive mutations in non-dividing cells 
, meiosis-associated hypermutability 
, hypermutability associated with repair of double-strand breaks (DSBs) 
and increased mutability associated with senescence in telomerase-deficient yeast 
. While the mechanisms remain to be explained, transient stretches of single-strand (ss) DNA have often been proposed as intermediates in hypermutation. Extended regions of ssDNA would be expected in association with DSBs and with dysfunctional uncapped telomeres, where many kilobases of ssDNA can be formed by 5′→3′ resection 
. The genome would be expected to be especially vulnerable to lesions in ssDNA since repair mechanisms such as base-excision repair (BER), nucleotide excision repair (NER) and post-replication repair (PRR) operate only in double-strand (ds) DNA. Restoration of damaged ssDNA to a ds-state would be expected to require translesion DNA synthesis (TLS) polymerases that are tolerant of lesions in a template and are often error-prone 
due to misincorporation during synthesis past lesions.
While error-prone restoration of damaged long stretches of ssDNA to dsDNA might explain many examples of transient hypermutability, this hypothesis has yet to be substantiated. Long tracts of ssDNA formed at DSBs and uncapped telomeres (and potentially in other cell contexts) are known to cause checkpoint responses, where the fate of the cell is determined by a balance of arrest, adaptation, recovery and repair processes 
. Direct estimates of extent of damage tolerance and amounts of damage-induced mutations in ssDNA formed in a chromosomal context are required to establish that damage in long ssDNA actually leads to hypermutability rather than to cell death. We, therefore, designed systems in the budding yeast Saccharomyces cerevisiae
for the generation of defined, persistent long stretches of transient ssDNA in the vicinity of a unique DSB or an uncapped chromosomal telomere. Lesions could be induced in the ssDNA before restoration to dsDNA. The consequences of restoration could be assessed with genetic reporters located within the regions that give rise to ssDNA. We demonstrate here that damage tolerance in stretches of long ssDNA formed at DSBs and uncapped telomeres results in extremely high frequencies of single and widely-separated multiple mutations caused by two major types of DNA damaging agents.