Demonstration that somatic and germinal nuclei are genetically equivalent. Using somatic cell nuclear transfer (SCNT),  Gurdon (Nobel Prize 2012) transplanted cell nuclei from mature intestinal tadpole cells into enucleated eggs, which developed into normal tadpoles. This contradicted the textbook dogma that adult cells are irrevocably assigned to their specific functions and cannot assume new ones. The egg was able to reprogram the introduced nucleus and direct its genes to switch from the duties of an intestinal cell to those appropriate to a developing egg.

Gurdon and Uehlinger replaced the cell nucleus of frog ova with frog intestinal nuclei to generate tadpoles, some of which became fertile adult male and female frogs.

In 2012 the Nobel Prize in Physiology or Medicine was awarded jointly to Sir John B. Gurdon and Shinya Yamanaka "for the discovery that mature cells can be reprogrammed to become pluripotent."  See also No. 256.12.

(Thanks to Juan Weiss for this reference and its interpretation.)