The siRNA-based knockdown of gene expression in human cells. Lentiviruses, RNA viruses that infect human cells, are used as vectors. A sequence encoding a hairpin homologous to the target gene is cloned into the lentiviral vector (red). In the host cell, the viral RNA is converted to double-stranded DNA by reverse transcriptase, and the DNA is integrated into the cellular genome. Transcription of the integrated gene produces a hairpin RNA that is converted to siRNA specific to the target gene.