CD45 controls immune responses by dephosphorylating several signaling molecules, including the Src family kinases (Lck and Fyn), the CD3z chain of the TCR, and the ZAP-70 kinase.[42,43] An imbalance between the activities of protein tyrosine kinases and phosphatases can lead to immunodeficiency.[43] CD45 controls the immune function by regulating lymphocyte survival, cytokine responses, and TCR signaling. Here, FYN is linked to immunodeficiency disease.