T cell α chain undergoes successive rearrangement until POSITIVE SELECTION or CELL DEATH INTERVENES
• Multiple Vα segments;60 Jα segmentsè higher chance for successive Vα to Jα rearrangements to take place • Continued rearrangement on both chromosomes can produce several different α chains with the same ß chain because expression of the T cell receptor is not in itself sufficient to shut off gene rearrangement.  3-4 days; ceases only when positive selection occurs or cell dies.
• expression of functional TcRα chain is NOT SUFFICIENT for allelic exclusion.
• POSITIVE SELECTION ensures that each T cell has only a single functional specificity