Epithelial cells require attachment to extracellular matrix (ECM) to suppress an 
apoptotic cell death program termed anoikis. Here we describe a nonapoptotic 
cell death program in matrix-detached cells that is initiated by a previously 
unrecognized and unusual process involving the invasion of one cell into 
another, leading to a transient state in which a live cell is contained within a 
neighboring host cell. Live internalized cells are either degraded by lysosomal 
enzymes or released. We term this cell internalization process entosis and 
present evidence for entosis as a mechanism underlying the commonly observed 
"cell-in-cell" cytological feature in human cancers. Further we propose that 
entosis is driven by compaction force associated with adherens junction 
formation in the absence of integrin engagement and may represent an intrinsic 
tumor suppression mechanism for cells that are detached from ECM.