After engraftment into wild-type B6 recipient mice, the expression of huCD20 by leukemia cells was diminished over time, whereas engraftment into huCD20 transgenic recipient mice did not affect the leukemia engraftment nor expression of huCD20. (A) Schema of how to distinguish engrafted E2A-PBX1 leukemia cells (CD45.2 homozygous) from recipient endogenous cells (CD45.1/CD45.2 heterozygous) using congenic markers (B) Human CD20-expressing E2A-PBX1 leukemia cells were engrafted into wild-type and huCD20 transgenic recipient mice. After 8 days, peripheral blood samples were collected, and the expression of huCD20 was measured in leukemia cells using flow cytometry. We observed a reduction in huCD20 expression on leukemia cells in the wild-type recipients (upper panel), whereas no change was seen in the huCD20 transgenic mice (lower panel). (C) The statistical analysis of panel B showed that the downregulation of huCD20 expression level is statistically significant in wild-type mice compared with that in huCD20-Tg mice.