In infants with a TlowBnormalNKnormal phenotype it is also important to ensure that the TCL is not due to a primary thymic defect (e.g., complete DiGeorge syndrome [21], FOXN1 deficiency [40], PAX1 defect [41] and Yamazaki Y et al., (manuscript submitted, 2019) which requires thymus transplantation as opposed to HCT. The gene discussed is PAX1; the disease is 22q11.2 deletion syndrome.