J. Clin. Invest. 109: 9–14 (2002). DOI: 10.1172/JCI200214794. represents an economical means by which cells can use the same signal transduction machinery to produce different responses in different developmental and/or cellular contexts. For example, active regulation of phosphotyrosine levels in resting cells may be essential for maintenance of a basal level of signaling necessary to keep the cell in a “primed” state, capable of rapid response if foreign antigen appears. Raising the threshold during T cell downregulation may be important for limiting the T cell response, allowing elimination of excess cells, and facilitating a return to homeostasis. Subsequently lowering the threshold in the surviving memory cells may enable a more rapid and robust response to reencounter of antigen. While many details remain to be elucidated, the reciprocal regulation of