CXCR4 and neoplasm: Interestingly, unlike Zlotnik's group, whohad suggested that CXCR4 expression was necessary for the movement of tumor cellsto secondary sites [25], Zeelenberg and colleagues found that CXCR4 expressionwas not required for migration of CT-26 colorectal tumor cells to the lungs,but rather for tumor expansion at secondary sites [71].