Sir2 (whose homolog in mammals is known as SIRT1, SIR2L1 or Sir2α) was the first gene of the sirtuin genes to be found. It was found in budding yeast, and, since then, members of this highly conserved family have been found in nearly all organisms studied. Sirtuins are hypothesized to play a key role in an organism's response to stresses (such as heat or starvation) and to be responsible for the lifespan-extending effects of calorie restriction.
Read more about Sir2: Nomenclature in Various Organisms, Method of Action and Observed Effects, Mammal Sirtuins