PDCD1 and leukemia: In contrast, PD-1 was upregulated upon the conversion to leukemic B cells in the BM, defined by the lack of CD19 expression in parallel with their emergence in the peripheral blood (PB) (Figure 1A and Figure S3A), indicating a loss of or marked reduction in Pax5 activity similar to what is found in human leukemia [24,36,37,38,39,49].