Trichoplax adhaerens is the simplest multicellular animal with tissue differentiation and somatic cell turnover. Like all other multicellular organisms, it should be vulnerable to cancer, yet there have been no reports of cancer in T . adhaerens or any other placozoan. We investigated the cancer resistance of T . adhaerens , discovering that they are able to tolerate high levels of radiation damage (218.6 Gy). To investigate how T . adhaerens survive levels of radiation that are lethal to other animals, we examined gene expression after the X-ray exposure, finding overexpression of genes involved in DNA repair and apoptosis including the MDM2 gene. We also discovered that T . adhaerens extrudes clusters of inviable cells after X-ray exposure. T . adhaerens is a valuable model organism for studying the molecular, genetic, and tissue-level mechanisms underlying cancer suppression.