Primary human ALL cells are phenotypically stable in serial xenotransplantation. The immunophenotype of 2 representative cases was determined in the same experiment on cryopreserved diagnostic cells and cells from multiple serial passage in NSG mice. Xenografts from 5 passages were analyzed for case VHR-03 (A) and 2 passages for case SR-09 (B). At least 10 000 viable cells were gated based on forward-side scatter properties, and histograms representing number of events versus signal intensity are shown.