Scientists have discovered a new way that 6-thioguanine (6-TG), a drug used for over 70 years to treat leukemia, works in the body. Researchers from the CeMM Research Center for Molecular Medicine in Austria, along with teams from the University of Oxford, the Weizmann Institute of Science, and the University of Dundee, found that a protein called NUDT5 plays an unexpected role in how cells respond to 6-TG. This discovery could lead to better ways of using thiopurine drugs, which are a class of medications used to treat various cancers and autoimmune diseases. Traditionally, NUDT5 was thought to function as a catalyst — a molecule that helps speed up chemical reactions in the cell. However, the new research shows that NUDT5 can also act as a molecular scaffold, helping to organize the cell's metabolic processes. This non-catalytic role appears to influence how sensitive cells are to 6-TG. The scientists tested this idea using a new technique called targeted protein degradation, which removes the protein entirely from the cell rather than just blocking its activity. This method allowed them to observe the effects of completely eliminating NUDT5, rather than just inhibiting it. The researchers found that simply blocking NUDT5's usual enzymatic activity did not significantly change how cells responded to 6-TG. However, when NUDT5 was entirely removed from the cells, the cells became more resistant to the drug’s toxic effects. This was confirmed through genetic experiments, which showed that NUDT5’s impact on drug sensitivity is not dependent on its traditional role as a catalyst. Instead, its structural role in organizing cellular processes seems to be more important in determining how the drug affects cells. The study also revealed a connection between NUDT5 and another protein, NUDT15, which is already known to influence how patients respond to thiopurine drugs. While NUDT15 makes cells more sensitive to 6-TG when it is absent, reducing NUDT5 makes cells more resistant. This suggests that the two proteins have opposing effects on drug sensitivity. The findings highlight that different mechanisms can influence how patients respond to thiopurine treatments. The study also shows the value of targeted protein degradation in uncovering biological functions that might be missed when using traditional enzyme inhibitors.