An isolated polynucleotide is provided, comprising a nucleic acid sequence encoding a soluble polypeptide which comprises an amino acid sequence of an N-terminus domain of CXCR4 and devoid of a CXCR4 extracellular domain selected from the group consisting of ECL1, ECL2 and ECL3, the soluble polypeptide being capable of binding SDF-1. Also provided are methods of using such a nucleic acid sequence such as for the treatment of cancer.