In March 2026, a paper in Molecular Cell revised the understanding of a cancer target whose inhibitors are already being tested in patients. That target is Pol theta, an enzyme nearly every cell has, there to patch breaks in damaged DNA. This repair job has made it a clinical-stage drug target for years: cells that can’t repair their DNA tend to die, and cancer cells, which rack up far more damage than normal cells, are especially vulnerable to losing that option. That’s the kind of established model a field builds whole drug programs on.

Those programs include trials enrolling patients right now, testing Pol theta inhibitors in BRCA-mutant tumors and others that have lost homologous recombination (HR), a different, generally more reliable way of repairing broken DNA than the one Pol theta handles. Losing HR alone doesn’t kill these tumor cells, since they still have Pol theta’s pathway to fall back on. Take that away too, and a cancer cell that’s already down to one option for fixing this kind of damage also loses its last chance of surviving it. Scientists call this synthetic lethality.

This year, two independent labs, researching different questions, found that Pol theta plays a different role than assumed.

The Model that Didn’t Hold

Scientists at Scripps Research, led by Xiaohua Wu, were looking at broken replication forks, one of the most common forms of damage that cancer cells accumulate under constant replication stress. Based on the existing model, break-induced replication (BIR) was understood to make the first attempt at repairing these breaks.

The microhomology-mediated version of that repair (fork-MMEJ), driven by the enzyme Pol theta, was assumed to act as a backup, stepping in only if BIR didn’t resolve things first. “We expected fork-MMEJ to follow the same rules as the version we’d studied before,” Wu said. “Finding that it didn’t follow the same rules meant that we were looking at something entirely new.”

The experiments kept turning up mutational signatures, telltale patterns left behind in the DNA, that didn’t fit what the established version of MMEJ was expected to leave behind. Fork-MMEJ was acting at the moment of the break itself. That’s not where the field expected to find it: before this, it was only known to patch breaks caused by something like radiation, damage with nothing to do with the act of copying DNA itself.

“We’ve uncovered a whole new dimension of how cancer cells cope with DNA damage at replication forks,” Wu said of the findings.

Two Labs, Same Timing

Seven months later, a team at Temple University, led by Tomasz Skorski, published a related finding in PNAS. PSF3, a protein that helps build the replication machinery, recruits Pol theta to forks that have stalled but not yet broken, where it helps restart replication or steps in to repair the break if the fork collapses anyway.

The Scripps finding showed Pol theta acting right at the break. This additional finding shows it acting even earlier than that, before a break even happens, not just immediately at one. Two independent labs, working on different questions, landed on the same conclusion: Pol theta gets involved earlier than the field assumed.

As Wu put it: “This changes how we think about when and why to use Pol theta inhibitors. If this pathway is acting at the very moment a fork breaks… then disrupting it could be far more consequential for cancer cells than we realized.” If forks broke only occasionally, timing wouldn’t matter much either way. But cancer cells force this to happen constantly, so a pathway that can act right at the break, rather than only sometimes as backup, changes the calculus on when an inhibitor should actually work.

What Doesn’t Change

Wu’s lab is already thinking about what comes next. “The more we understand about the factors involved in this pathway, the more potential targets we have,” she noted. Pol theta won’t be the last target whose model turns out to be incomplete, and more potential targets means more need for the kind of diverse assays that can study them, which is exactly what our application resources for DNA damage repair are built to offer.

The targets change. The tools evolve. What stays constant is the need for confirmation. The measurement side is where we live.

Resources

1. Li, S. et al. (2026) Microhomology-mediated end joining acts directly on replication forks to repair single-ended double-strand breaks. Mol. Cell 86, 1230. DOI: 10.1016/j.molcel.2026.02.016. https://www.cell.com/molecular-cell/fulltext/S1097-2765(26)00130-9
2. Vekariya, U. et al. (2026) PSF3 recruits Polθ to stalled replication forks to promote DNA repair. Proc. Natl. Acad. Sci. U.S.A. 123, e2613699123. DOI: 10.1073/pnas.2613699123. https://www.pnas.org/doi/10.1073/pnas.2613699123
3. Scripps Research (2026) Scientists reveal a new way cancer cells survive DNA damage. Published April 7, 2026. https://www.eurekalert.org/news-releases/1123129

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Elise Johnson

Elise Johnson is a Marketing Copywriter at Promega who helps turn complex science into stories that move readers from curiosity to understanding. With a background in education, she’s drawn to the intersection of language, learning, and science communication. Outside of work, Elise enjoys being outdoors, reading, and indulging her curiosity.

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