Activation of the B-cell receptor on CLL cells induces a characteristic gene expression signature and phosphorylation of SYK. (A) Gene expression changes in UM-CLL (n = 4) and M-CLL (n = 4) samples 6 hours after in vitro IgM cross-linking. Sixty-one genes showing a > 2-fold change between unstimulated and stimulated cells (P < .001) were identified and constitute a “CLL-BCR gene signature.” Gene expression changes in response to BCR engagement is depicted in a heat map according to the scale shown. Genes are sorted top to bottom for the ratio between the average up-regulation in UM-CLL compared with M-CLL cells. (B) Phosphorylation of SYK (p-SYK) after in vitro IgM cross-linking was assessed by flow cytometry in cells gated on CD3⁻SYK⁺IgM⁺ at the indicated time points. (C) Mean and SD of the percentage of pSYK-positive cells in CLL subtypes (UM-CLL n = 14, M-CLL n = 13) and normal B cells (control n = 11) are shown. Differences between groups was assessed by Student t test (P = .013 for UM-CLL vs M-CLL at 45 minutes; all other comparisons were not significant).