This year's Nobel Prize in physiology or medicine has been awarded to the United States scientists Victor Ambros and Gary Ruvkun, judges at the Karolinska Institute in Stockholm, Sweden, announced on Monday.

The panel of judges, known as the Nobel Assembly, selected the winners in acknowledgment of their discovery of microRNA and its role in post-transcriptional gene regulation.

They said Ambros and Ruvkun had, through their discovery, increased the understanding of gene regulation in animals and plants.

"Their groundbreaking discovery revealed a completely new principle of gene regulation that turned out to be essential for multicellular organisms, including humans," the assembly said in a statement.

Scientists have been working for decades to improve the understanding of how gene activity is regulated because, when it goes wrong, it can cause serious diseases, including autoimmunity problems, cancer and diabetes.

The Nobel Assembly said Ambros and Ruvkun made their breakthrough while studying mutant strains of a roundworm called Caenorhabditis elegans.

The pair initially worked on the project together in the same laboratory before Ambros moved to Harvard University and Ruvkun to Massachusetts General Hospital, where they continued their collaboration at a distance.

The Nobel Prize in physiology or medicine has been awarded 115 times, to 229 prize winners, or laureates as they are known.

The prize in medicine was the first in a series of Nobel Prize announcements set for this week. The Nobel Prize for physics will be announced on Tuesday, while prizes for chemistry, literature and peace will be unveiled on Wednesday, Thursday and Friday.

Laureates will collect a medal and a cash award of 11 million Swedish krona ($1.06 million) left for the purpose by the annual competition's founder, the Swedish inventor Alfred Nobel. While laureates are announced this week, they will pick up their prizes at a ceremony on Dec 10, the anniversary of Nobel's death in 1896.

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