An international team led by Goethe University Frankfurt has identified an intracellular sensor that monitors the quality of so-called MHC-I molecules, which help the immune system recognize and kill harmful cells, including tumor cells. The sensor ensures that defective MHC-I molecules remain inside the cell, where they are eventually degraded. Surprisingly, a lack of this quality assurance can lead to more MHC-I molecules reaching the surface of cancer cells, triggering a stronger immune response against the tumor.