Figure 2.

Approaches to endogenous expression of siRNAs in mammalian cells. (a) Sense and antisense strand of the siRNA duplex are expressed from separate promoters. (b) siRNA duplex is expressed as a stem-loop structure (small hairpin RNA [shRNA]) from a single promotor. Sense and antisense strands are separated by a loop-forming spacer. The construct is further processed by Dicer within the cell to form a functional siRNA. In both cases transcription is terminated by six consecutive thymidine residues.