Victor Ambros and Gary Ruvkun were awarded the Nobel Prize in Physiology or Medicine on Monday for the discovery of microRNA, a tiny class of RNA molecules that play a crucial role in determining how organisms mature and function — and how they sometimes malfunction.

Working with curious, millimeter-size roundworms of the species Caenorhabditis elegans, the two laureates’ discovery revealed a new principle of gene regulation that is crucial for the development and health of multicellular organisms, including humans, Nobel Prize officials said.

Dr. Ambros is a professor of natural science at the UMass Chan Medical School in Worcester, Mass., and Dr. Ruvkun is a professor of genetics at Harvard Medical School and a researcher at Massachusetts General Hospital.

Gene regulation determines differences between types of cells, and if it goes off track it can lead to diseases such as cancer, diabetes or autoimmunity, the Nobel committee said. Researchers now know that the human genome provides instructions for over 1,000 forms of microRNA.

“That opened up a whole new understanding of how diseases happen, which means that we have new possibilities for reversing them,” said Jon Lorsch, director of the National Institute of General Medical Sciences and an institute director at the National Institutes of Health.

Treatments based on microRNA are in clinical trials for heart disease, cancer and neurodegenerative disease.