Patients who achieved bone marrow engraftment of cord blood-derived progenitor 
cells provided an opportunity to examine the expression of fetal Hb by neonatal 
hematopoietic progenitors in a postneonatal host. Cord blood cells from 
histocompatible siblings were successfully transplanted in two children with the 
Fanconi anemia syndrome. One of the transplant donors had heterocellular 
hereditary persistence of fetal Hb, apparently due to gamma-globin gene 
triplication; the other donor was hematologically normal. The G gamma/A gamma 
ratio of the patient who received his transplant from the donor with hereditary 
persistence of fetal Hb was markedly elevated, similar to that of the transplant 
donor's cord blood, and this ratio remained elevated in subsequent months. In 
the other child, the G gamma/A gamma ratio immediately after her transplant was 
typical of the normal newborn, and over the next several months it reverted to 
the adult pattern. Globin synthesis studies performed shortly after engraftment 
demonstrated ratios of fetal Hb/adult Hb synthesis in both patients that were 
typical of those of normal newborns. Over the next several months, both patients 
converted to the adult pattern. Fetal Hb to adult Hb switching in these patients 
seemed to follow a temporal sequence intrinsic to the transplanted neonatal 
progenitor cells, without discernible influence of postneonatal environmental 
factors. The program for Hb switching seems to be an inherent feature of 
neonatal hematopoietic progenitor cells.