CXCR4 and neoplasm: Further, the relationship between CXCR4 and tumor initiating cell markers CD133/CD44 is reciprocal: CXCR4+ cells have a higher percentage of CD133+/CD44+ than CXCR4− cells and CD133+/CD44+ cells contain more CXCR4+ cells than CD133−/CD44− cells.