Within our field, improvement in fluorescence-activated cell sorting (FACS) and molecular technologies has led to various types of correlative studies that imply the developmental sequence and subsequent emigration of thymic-lymphocyte subsets. Unfortunately, the implied conclusions are often accepted unequivocally by most of the immunology community. In fact, direct demonstration of precursor progeny relationships by specific cell marking within the thymus, or specific delivery of purified cells at a particular stage of isolation back into the thymus, are the only methods that reproducibly identify cell stages and intermediates (1 –11 ).
