RUNX1 and precursor B-cell acute lymphoblastic leukemia: In agreement with the delayed exposure hypothesis, epidemiologic studies suggest that exposures leading to immune stimulation (i.e., breastfeeding, daycare attendance, contact with pets, allergies) during early childhood are correlated with reduced development of the most common B-ALL subtypes (ETV6-RUNX1+ and high-hyperdiploid B-ALL)16–21.