A possible side-effect of cancer radiotherapy is that irradiated cells or tissues release factors that can affect other tissues and cells that have not been exposed to radiation. This concept, known as radiation-induced bystander effect (RIBE), is well established, but its molecular mediators remain less clear. Studying nematode worms, Ding Xue and colleagues report the identification of an enzyme cysteine protease cathepsin B (CPR-4) as a RIBE factor. They also provide insights into how CPR-4 exerts its effects following secretion in response to exposure of the animals to ultraviolet or ionizing radiation. The protein seems to cause inhibition of cell death and increased embryonic lethality in unexposed parts of the animal through a mechanism that is regulated by the homologue of the tumour suppressor protein p53.
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