Cell-in-cell, a phenomenon characterized by one or more viable cells entering
actively into another cell, was observed more than a century and has only attracted more
attention in recent years and is becoming a new hot topic in the biological field, owing its
biological significance in evolutionary as well as physiological and pathological relevance in
development, homeostasis and diseases. In this paper we focus on the diversity,
evolutionary conservatism and clinical implication of cell-in-cell as well as latest opinions on
the research strategies. Based on the findings from our laboratory and other research groups
three working models of cell-in-cell are also proposed.