The chemokine receptor, CXCR4, and its specific ligand, CXCL12, have been proven to regulate the directional trafficking and invasion of breast cancer cells to sites of metastases, and similar phenomena have also been identified in many malignant tumors that aberrantly overexpress CXCR4. Therefore, blocking the interaction between CXCR4 and CXCL12 is considered a possible approach to efficiently prevent cancer metastasis. Employing a cellular phenotypic knockout strategy based on intrakines, we developed a novel recombinant chimeric protein, TAT/54R/KDEL, which contains three distinct functional domains: CXCL12/54R, a mutant of CXCL12 with CXCR4 antagonism, as well as HIV-derived TAT (47-57) and an endoplasmic reticulum retention four-peptide sequence KDEL that links at its NH(2) and COOH termini, respectively. Using the MOLT-4 cell line, which expressed CXCR4 highly and stably in vitro, we determined that TAT/54R/KDEL was able to efficiently transfer into the endoplasmic reticulum of tumor cells, where it specifically binds to the newly synthesized CXCR4 and prevents the latter from reaching the surface. Chemotaxis assays showed that the cells treated with TAT/54R/KDEL failed to migrate toward CXCL12. Furthermore, we observed that the systemic treatment of TAT/54R/KDEL could impair lung metastasis in a highly metastatic mammary cancer cell line, 4T1 cells, with the decrease of CXCR4 on their membrane. Our results suggest that the phenotypic knockout strategy of CXCR4 using a novel recombinant protein TAT/54R/KDEL might be a possible approach for inhibiting relative tumor metastasis mediated by CXCR4/CXCL12 interaction.