The discovery of Sema 4D can be dated to 1992, when Boumsell＇s teamreported a novel 150-kDa glycoprotein dimer. It was expressed on T lymphocytes that had been activated with CD3 mono- donal antibody or phytohaemagglutinin.1 Hence, this molecule has been designated CD100 by the International Workshop on Human Leukocyte Differentiation Antigen. In 1996, 3 years after the discovery of the semaphorin family,2 the CD100 gene was cloned and identified as the first sema- phorin with immune system functions.3 Unlike the prototypical semaphorins, which are neuronal chemorepellents, Sema 4D/ CD100 together with Sema 3A, Sema 4A,
