Yin et al engineered a Bcor mutation in BM cells or fetal liver cells from NP23 Tg mice and injected them into lethally irradiated mice. All the recipient mice developed leukemia with B-1 progenitor phenotype and the same Igh usage with B-1 cells. B-1 pro, B-1 progenitors; B-2 pro, B-2 progenitors.