The impairment of immune cell functions in cancer patients may result from impairment of signal transduction. Recent studies have demonstrated altered expression and function of signal transduction molecule CD3 zeta in T and natural killer cells in patients with neoplastic diseases. Tumor infiltrating lymphocytes are more affected than peripheral blood lymphocytes. CD3 zeta expression is more prominent in advanced stage of the disease and correlates with reduced proliferative response, IFN-gamma, IL-2, TNF-alpha production.