CXCR4^E343K is a gain-of-function mutation: analysis in transfected K562 cells. (A,C) Expression of CXCR4 variants on untransfected K562 cells (gray area) or K562 cells transfected transiently (A) or stably (C) with CXCR4^WT (solid line), CXCR4^R334X (dashed line), and CXCR4^E343K (dotted line). X-axis shows the mean fluorescence intensity of CXCR4 expression. (B) Peak calcium-flux responses of K562 cells transiently transfected with CXCR4^WT, CXCR4^R334X, and CXCR4^E343K in response to the indicated doses of CXCL12. *P < .05; **P < .01; and ***P < .001 for the comparison between CXCR4^WT and either CXCR4^R334X or CXCR4^E343K. Data are summarized as the means ± SEM from 4 independent experiments with 3 replicates each. (D) The peak calcium-flux response of K562 cells stably transfected with CXCR4^WT, CXCR4^R334X, and CXCR4^E343K in response to 50nM CXCL12 stimulation. *P < .05 and **P < .01 for the comparison between CXCR4^WT and CXCR4^E343K or CXCR4^R334X.