In an age- and GVHD-matched comparison of children and young adults between UCB [median age 12.6 years (3–34.6)] and matched sibling recipients, TREC and CD4+CD45RO− naïve T cells were significantly higher, whereas CD8+ activated and memory T cells were lower at 2 years in UCB as compared to matched sibling donor group, indicating efficient thymopoiesis in UCBT [48]. Here, CD4 is linked to graft versus host disease.