Single cell suspensions of freshly dissected thymuses were obtained and either mechanically suspended or enzymatically digested as previously described5 (link), 74 (link) and counted using the Z2 Coulter Particle and Size Analyzer (Beckman Coulter, USA). For studies sorting rare populations of cells in the thymus, multiple identically treated thymuses were pooled so that sufficient number of cells could be isolated; however, in this instance separate pools of cells were established to maintain individual samples as biological replicates.