PCNA and B-cell chronic lymphocytic leukemia: In order to directly compare the proliferation rate of CLL cells in the murine spleen to that in the corresponding patient's LN, we analyzed xenografted CLL cells from three patients that have previously donated PB and LN.3 In purified CLL cells we measured expression of three genes that are upregulated in proliferating cells and that we have previously shown to be expressed in the human LN.3 The mRNA level for each gene (CDT1, PCNA, and RRM2) was determined by quantitative RT-PCR (Table S1).