Cellular DNA is under constant challenge by exogenous and endogenous genotoxic 
stress, which results in both transient and accumulated DNA damage and genomic 
instability. All cells are equipped with DNA damage response pathways that 
trigger DNA repair, cell cycle arrest, and, if need be, apoptosis, to eliminate 
DNA damage or damaged cells. The consequences of these processes for stem cells 
can be profound: diminution in stem cell pools, or, because of altered gene 
expression, an increased chance for stem cell differentiation or malignant 
transformation. Furthermore, a number of DNA repair abnormalities are linked to 
premature aging syndromes, and these are associated with defects in the stem 
cell population. The specific DNA repair systems for which there are data 
regarding the impact of repair defects on stem cell function include 
O(6)-alkylguanine DNA alkyltransferase, nucleotide excision repair, base 
excision repair, mismatch repair, non-homologous DNA end-joining Fanconi's 
anemia protein complex, and homologous recombination. It has recently become 
clear that deficiencies of these processes are associated not only with cancer 
and/or aging but also with stem cell defects. This discovery raises the 
possibility of a link between aging and stem cell dysfunction. In this review, 
we provide evidence for a link between DNA repair systems and the maintenance 
and longevity of stem cells.