ConclusionAmong the open questions

ConclusionAmong the open questions on the relationships between cell-in-cell phenomena and cancer, we believe that one — are cell-in-cell structures a hallmark of malignant cancers? — has been answered. Cell-in-cell structures have been reported for decades in clinical specimens4, and their presence has been used as a marker of malignancy55,56 with prognostic significance46,57,58. Moreover, researchers have now identified this cell behaviour occurring in numerous cancer types in humans or animal models (Box 1). Cell behaviour leading to engulfment of cells is therefore a common event occurring during malignant transformation, and we consider here that this is a hallmark of cancer cell behaviour.If cell-in-cell processes