Overexpression of sirtuins (NAD(+)-dependent protein deacetylases) has been 
reported to increase lifespan in budding yeast (Saccharomyces cerevisiae), 
Caenorhabditis elegans and Drosophila melanogaster. Studies of the effects of 
genes on ageing are vulnerable to confounding effects of genetic background. 
Here we re-examined the reported effects of sirtuin overexpression on ageing and 
found that standardization of genetic background and the use of appropriate 
controls abolished the apparent effects in both C. elegans and Drosophila. In C. 
elegans, outcrossing of a line with high-level sir-2.1 overexpression abrogated 
the longevity increase, but did not abrogate sir-2.1 overexpression. Instead, 
longevity co-segregated with a second-site mutation affecting sensory neurons. 
Outcrossing of a line with low-copy-number sir-2.1 overexpression also abrogated 
longevity. A Drosophila strain with ubiquitous overexpression of dSir2 using the 
UAS-GAL4 system was long-lived relative to wild-type controls, as previously 
reported, but was not long-lived relative to the appropriate transgenic 
controls, and nor was a new line with stronger overexpression of dSir2. These 
findings underscore the importance of controlling for genetic background and for 
the mutagenic effects of transgene insertions in studies of genetic effects on 
lifespan. The life-extending effect of dietary restriction on ageing in 
Drosophila has also been reported to be dSir2 dependent. We found that dietary 
restriction increased fly lifespan independently of dSir2. Our findings do not 
rule out a role for sirtuins in determination of metazoan lifespan, but they do 
cast doubt on the robustness of the previously reported effects of sirtuins on 
lifespan in C. elegans and Drosophila.