Taken all together, biotinylated JM173 peptides mediate cell type-selective delivery of the modular bioconjugate (JM173)3-Avi-C3 into CXCR4-expressing cancer cells and the release of the therapeutic cargo protein C3 into their cytosol to down-modulate Rho-/actin-dependent growth of cancer cells. The gene discussed is CXCR4; the disease is cancer.