Durability and efficiency of selection depends on the cell type selected for. In vivo selection of hematopoietic cells can occur at the level of stem cells, which persist for life, or at the level of progenitor cells with limited self-renewal capacity. Data from animal models of in vivo selection suggest that in vivo selection at the stem cell level leads to durable increases in gene marking, whereas selection at more mature stages of the hematopoietic hierarchy is short-lived. Selection at the level of a non-stem cell would likely need to be applied repeatedly or continuously. An exception may be diseases of lymphoid immunity, where the situation is complicated by the existence of long-lived mature cells (ie, memory cells).