CXCR2 and neoplasm: Moreover, the tumors that did not lose p21 expression (41 (54.67%) of 75) may carry a mutation in a downstream mediator of hPTTG1/CXCR2/p21 signaling to evade senescence, or the hPTTG1/CXCR2/p21-expressing cells may represent only a portion of a tumor composed of heterogeneous cell types.