A patient with recurrent bacterial and viral infections is
referred to an internist who determines that, while there are
normal numbers of T cells in the circulation, the T cells are
extremely unresponsive to all antigenic stimuli. Further testing
determines that the T lymphocytes are severely deficient in the
ability to signal following exposure to antigen. Which of the
following cell surface molecules is most likely to be mutated,
making these lymphocytes unable to generate and transmit activation
signals upon...