Signals delivered through the b/gp33 (pre-TCR) and T-cell receptor ab control proliferation and differentiation of thymocytes at two distinct control points of T cell maturation. Interaction between T-cell receptor (TCR) and peptide/MHC complex induce signaling pathways leading to activation of T cell. Signal transduction involves CD3z phosphorylation by Lck tyrosine kinase and activation of ZAP-70 which regulates signaling pathways through PKC, Ca++ and Ras/Raf kinase cascade. Appropriate response of cell is preceded by integration of different signals in the nucleus.