This illustration explores the potential of short interfering RNA (siRNA) sequences for blocking HIV infection. Complementary sequences of siRNA can block expression of the human CD4 gene - which normally expresses a cell surface receptor used by HIV to gain access to the cell - by about 75%, effectively preventing infection. Other siRNA targets include structural proteins of HIV itself. Illustrated is an siRNA duplex (purple) binding to a newly synthesized viral transcript, blocking assembly of new virus.