We overcome the problem of knocking down highly expressed gene function by combining intracellular single chain antibodies (intrabodies) and short interfering RNA (siRNA) to the same gene product. The chemokine receptor CXCR4 is expressed at high levels on T cells and is the co-receptor for pathogenic HIV-1s (termed X4-tropic). Furthermore, it has been shown that HIV-1 requires low amounts of CXCR4 for X4-tropic viral entry (Konopka and Düzgüne AIDS Res Hum Retro, 2002). In contrast to CCR5, due to the high levels of CXCR4, it has been more difficult to provide protection from HIV-1 challenge using conventional siRNA strategies. To address this issue a CXCR4 siRNA and intrabody gene combination HIV-1 vector has been generated. The CXCR4 intrabody is specific for the C-terminal domain of CXCR4 and is coupled to the KDEL endoplasmic retention signal and CXCR4 siRNA targets mRNA nucleotides 331–349 (Martinez, et al. AIDS, 2002). Using the myelomonocytic THP-1 cell line, a line that highly expresses cell surface CXCR4, it was found that CXCR4 intrabody or siRNA vectors mediated a 50% reduction in cells expressing CXCR4 as assessed by flow cytometry. In contrast, the level of CXCR4 cell knockdown increased to 82% using the CXCR4 siRNA and intrabody combinatorial vector. Moreover, the CXCR4 mean fluorescent intensity in cells containing the combination vector resembled the mean fluorescent intensity of cells analyzed with isotype control antibodies, i.e., little detectable CXCR4 expression as judged by flow cytometry. Similar CXCR4 knockdown trends were seen in primary CD4+ T-cells expressing the combinatorial vector as compared to vectors containing the intrabody gene or siRNA alone. HIV-1 X4-tropic challenge studies using a multiplicity of infection of 1 have shown that THP-1 cells expressing the combinatorial vector are resistant to infection (HIV-1 p24 production < 3 ng/ml), whereas THP-1 cells expressing the siRNA or intrabody gene vector were susceptible to infection (HIV-1 p24 > 15–35 ng/ml). These findings demonstrate that targeting both the message and protein product from the same gene is vastly superior to the targeting of either message or protein. The combinatorial vector is currently being tested in humanized mice for efficacy. Lastly, targeting a highly expressed cellular product at the message and protein level may prove beneficial for efficient gene knockdown outside the HIV-1 research and therapy field, such as for metastatic cancers associated with CXCR4/SDF-1 upregulation.