Roberto Testi*#, Daniele D'Ambrosio*, Ruggero De Maria* and Angela Santoni*

#Department of Experimental Medicine and Biochemical Sciences, University of Rone "Tor Vergata" and *Department of Experimental Medicine, University of Rome "La Sapienza"

Corresponding author: Roberto Testi

Initially described as an antigen restricted to recently activated lymphoid cells, CD69 has been now detected on the surface of all hematopoietically derived leukocytes. Crosslinking of CD69 generates intracelluar signals, resulting in a variety of cellular end responses, in all cell lineages studied, both in the mouse and in the human system. A specific ligand has not been identified yet, thus a definite functional identity for CD69 is still missing. However, as discussed by Roberto Testi and his collegues, its broad expression and conserved ability to generate intracellular signals suggest a general role for the CD69 receptor in the biology of hematopoietic cells.