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Transmissible cancers are infectious malignant cell clones that spread among individuals through transfer of living cancer cells. Several such clones have been identified in various species of marine bivalve molluscs, including mussels, clams and cockles. These parasitic cell lineages cause a leukaemia‐like disease called disseminated neoplasia, and are presumed to pass between hosts by ingestion of water‐borne cancer cells during filter feeding. Although occasional cases of transmissible cancer had previously been identified in mussels of the genus Mytilus in Europe, the number of distinct clones affecting these animals, and their prevalence, was unknown. In this issue of Molecular Ecology, Hammel et al. (2021) present findings from a large‐scale screen for transmissible cancer across 5907 European Mytilus mussels. Using a genotyping approach, Hammel et al. searched for signal of genetic chimerism, which can arise due to infection by transmissible cancer cells. The screen detected a previously identified globally distributed mussel transmissible cancer at very low prevalence, and found no evidence of additional contagious clones. A parallel histological screen additionally revealed low prevalence of a nontransmissible form of disseminated neoplasia. By quantifying the burden of disseminated neoplasia in European mussel populations, this study provides strong foundations for future work investigating the origins, evolution and impacts of transmissible cancers in mussels. [ABSTRACT FROM AUTHOR]