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The cancer research community experienced a sea change in 2013 as a strategy, decades in the making, finally cemented its potential. Promising results emerged from clinical trials of cancer immunotherapy, in which treatments target the body's immune system rather than tumors directly. The new treatments push T cells and other immune cells to combat cancer--and the editors of Science believe that such approaches are now displaying enough promise to top their list of the year's most important scientific breakthroughs.

This annual list of groundbreaking scientific achievements, selected by Science and its international nonprofit publisher, AAAS, also includes major breakthroughs in solar cell technologies, genome-editing techniques and vaccine design strategies, to name a few. The top-10 list appears in the 20 December issue of the journal along with a related news feature and a multimedia component.

Cancer immunotherapy clinched the #1 spot on the list because, although its ultimate impact on the disease is unknown, recent results are highlighting its success so far.

"This year there was no mistaking the immense promise of cancer immunotherapy," said Tim Appenzeller, chief news editor of the journal Science. "So far, this strategy of harnessing the immune system to attack tumors works only for some cancers and a few patients, so it's important not to overstate the immediate benefits. But many cancer specialists are convinced that they are seeing the birth of an important new paradigm for cancer treatment."

Many of today's advances in cancer immunotherapy can be traced back to the late 1980's, when French researchers identified a receptor on T cells, called CTLA-4. James Allison discovered that this receptor prevented T cells from attacking invaders with their full force. In the mid-1990's Allison showed that blocking CTLA-4 in mice could unleash T cells against tumor cells in the animals, shrinking them dramatically.

In the meantime, Japanese researchers identified another "brake" on T cells known as PD-1. Clinical trials involving this receptor began in 2006, and preliminary results in small groups of patients appear to be promising.

Another area of interest involves genetically modifying T cells to make them target tumors. In 2011 this strategy, known as chimeric antigen therapy, or CAR therapy, electrified the cancer research field, and it's now the subject of numerous clinical trials, particularly in blood cancers.

Accordingly, many pharmaceutical companies that wanted nothing to do with immunotherapy several years ago are now investing heavily.

There's still lots of uncertainty regarding how many patients will benefit from these therapies, most of which remain experimental--and for which forms of cancer they will work best. Scientists are busy trying to identify biomarkers that might offer answers, and thinking of ways to make treatments more potent. But a new chapter in cancer research and treatment has begun and the journal Science acknowledges this fact by recognizing cancer immunotherapy as the most significant scientific breakthrough of 2013.

The journal's list of nine other groundbreaking scientific achievements from the past year follows.

CRISPR: This gene-editing technique was discovered in bacteria, but researchers now wield it as a scalpel for surgery on individual genes. Its popularity soared this year as more than a dozen teams of researchers used it to manipulate the genomes of various plant, animal and human cells.

Perovskite Solar Cells: A new generation of solar-cell materials, cheaper and easier to produce than those in traditional silicon cells, garnered plenty of attention this past year. Perovskite cells are not as efficient as commercial solar cells yet, but they are improving very quickly.

Structural Biology Guides Vaccine Design: This year, researchers used the structure of an antibody to design an immunogen--the main ingredient of a vaccine--for a childhood virus that hospitalizes millions each year. It was the first time that structural biology led to such a powerful tool for fighting disease.

CLARITY: This imaging technique, which renders brain tissue transparent and puts neurons (as well as other brain cells) on full display, changed the way that researchers look at this intricate organ in 2013.

Mini-Organs: Researchers made remarkable progress growing mini human-like "organoids" in vitro this year. These included liver buds, mini-kidneys and tiny brains. Such miniaturized human organs may prove to be much better models of human disease than animals.

Cosmic Rays Traced to Supernova Remnants: Although originally detected 100 years ago, scientists haven't been sure where the high-energy particles from outer space known as cosmic rays come from. This year, they finally tied the rays to debris clouds left by supernovae, or exploding stars.

Human Cloned Embryos: Researchers were able to derive stem cells from cloned human embryos this year after realizing that caffeine plays an important role in the process, stabilizing key molecules in delicate human egg cells.

Why We Sleep: Studies with mice showed that the brain cleans itself--by expanding channels between neurons and allowing more cerebrospinal fluid to flow through--much more efficiently during sleep. The finding suggests that restoration and repair are among the primary purposes of catching Z's.

Our Microbes, Our Health: Research on the trillions of bacterial cells that call the human body home made it clear how much these microbes do for us. "Personalized" medicine will need to take these microbial tenants into account in order to be effective.

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Science's 2013 Breakthrough of the Year feature, along with news, videos and a podcast will be available at news.sciencemag.org/breakthrough-of-the-year-2013 and http://scim. ag/ Breakthrough13 after the embargo lifts.

The American Association for the Advancement of Science (AAAS) is the world's largest general scientific society, and publisher of the journal, Science as well as Science Translational Medicine and Science Signaling. AAAS was founded in 1848, and includes some 261 affiliated societies and academies of science, serving 10 million individuals. Science has the largest paid circulation of any peer-reviewed general science journal in the world, with an estimated total readership of 1 million. The non-profit AAAS is open to all and fulfills its mission to "advance science and serve society" through initiatives in science policy; international programs; science education; and more. For the latest research news, log onto EurekAlert!, the premier science-news Web site, a service of AAAS.