OBJECTIVE: There is a broad therapeutic potential for the application of small interfering RNAs (siRNAs). However, one has to ensure that siRNAs act specifically, only targeting the expression of one gene. Off-target effects raised by the sense strand have to be eliminated. METHODS AND RESULTS: We examined a particular bidirectional siRNA molecule, able to knockdown intercellular adhesion molecule-1 (ICAM-1) and tumor necrosis factor receptor-1 (TNFR-1) by the sense or antisense strand, respectively. Transfection of human venous endothelial cells with an unmodified siRNA molecule led to equal silencing of ICAM-1 and TNFR-1. In contrast, modified siRNA was able to knockdown ICAM-1 and TNFR-1 separately, with only the antisense strand. DISCUSSION: We found the modified siRNAs to inhibit off-target effects originated by the sense strand. Our approach demonstrates one possibility to modify siRNAs before starting a clinical approach to eliminate off-target effects.