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High choline kinase-α (Chk-α) expression is frequently observed in cancer cells, making it a novel target for pharmacological and molecular inhibition. As inhibiting agents are delivered systemically, it is important to determine Chk-α expression levels in endothelial cells that line both normal and tumor vasculature, and the effect of Chk-α downregulation on these cells. Here, we characterized Chk-α expression and the effect of its downregulation in human umbilical vein endothelial cells (HUVECs) relative to MDA-MB-231 human breast cancer cells. We used small interfering RNA (siRNA) to downregulate Chk-α expression. Basal mRNA levels of Chk-α were approximately three-fold lower in HUVECs relative to MDA-MB-231 breast cancer cells. Consistent with the differences in Chk-α protein levels, phosphocholine levels were approximately 10-fold lower in HUVECs relative to MDA-MB-231 cells. Transient transfection with siRNA-Chk resulted in comparable levels of mRNA and protein in MDA-MB-231 breast cancer cells and HUVECs. However, there was a significant reduction in proliferation in MDA-MB-231 cells, but not in HUVECs. No significant difference in CD31 immunostaining was observed in tumor sections obtained from mice injected with control luciferase-short hairpin (sh)RNA or Chk-shRNA lentivirus. These data suggest that systemically delivered agents that downregulate Chk-α in tumors will not affect endothelial cell proliferation during delivery, and further support the development of Chk-α downregulation as a cancer-specific treatment., (Copyright © 2013 John Wiley & Sons, Ltd.)