The 2009 Nobel Prize in Physiology or Medicine was awarded for which of the following?

For discoveries of odorant receptors and the organization of the olfactory system

For discovery of RNA interference - gene silencing by double-stranded RNA

For the discovery of how chromosomes are protected by telomeres and the enzyme telomerase

For discoveries of principles for introducing specific gene modifications in mice by the use of embryonic stem cells

For the discovery of human immunodeficiency virus