Introduction
In its infancy, RNA interference (RNAi) was simply an intriguing curiosity; now it has leapt to the forefront of science (Couzin, 2002). The growth spurt of RNAi-related studies has been partly pragmatic – RNAi clearly had the potential to evolve into an incredibly powerful technology for manipulating gene expression in diverse cell types, and to a great extent this potential has been realized. Moreover, as a physiological phenomenon, RNAi represents a fascinating and previously unrecognized level of cellular regulation. RNAi is essential for silencing of heterochromatin (Reinhart and Bartel, 2002; Volpe et al., 2002; Schramke and Allshire, 2003; Volpe et al., 2003) and gene expression via methylation (Grant, 1999; Jones et al., 1999), for antiviral defense, at least in plants (Baulcombe, 1999; Grant, 1999; Ratcliff et al., 1999), for controlling the expression of transposable elements and repetitive sequences (Ketting et al., 1999; Tabara et al., 1999; Ambros et al., 2003a&b; Sijen and Plasterk, 2003), and for proper embryonic development (Grishok et al., 2001; Hütvagner et al., 2001; Ketting et al., 2001; Knight and Bass, 2001; Bernstein et al., 2003; Houbaviy et al., 2003).
This chapter focuses on Dicer, an RNAse III family enzyme essential for sequence-specific gene suppression. We begin with an overview of the discovery of Dicer and its implication in RNAi, followed by a discussion of its hypothesized roles in vivo.