RUNX1 and leukemia: If the frequencies of PGF detected in UCB samples of healthy newborns were really so high, i.e. ∼4% TEL-AML1, ∼6% BCR-ABL p190 and 0.75% MLL-AF4, with a defined frequency of children leukemia (1∶10,000), it would suggest that the presence of PGF in a newborn is not as important as the number of HSC/progenitor cells bearing a fusion transcript in the particular UCB which depends on the time during fetal development when this chromosomal rearrangement arose.