p59fYn represents one of the protein tyrosine kinase expressed in T lymphocytes whose precise function during T cell activation is poorly understood. To gain insight into the mechanisms by which Fyn is involved in T cell signaling, we propose to purify and clone phosphoproteins that interact with the kinase through its SH2 domain. Subsequently, we will investigate whether the Fyn complex undergoes modifications during T cell activation. Additionally, in order to further assess the critical role of Fyn and its associated proteins, we will also analyze the Fyn complex under pathophysiological conditions, e.g. in T lymphocytes isolated from leukemic patients. The realization of this project could contribute to better understanding the role of Fyn in T cell signaling as well as the mechanisms involved in the developement of T cell lymphomas or leukemias.