Metzger et al. have found that a leukaemia-like type of cancer in soft-shell clams that is causing substantial population loss worldwide is transmissible. The authors found that the genotypes of the clam leukaemic cells were different from their hosts and nearly identical to the genotypes of other neoplastic cells in clams from dispersed locations. This suggests that the cancer is spreading as a clonal transmissible cell derived from a single original clam.