Members of the serine/arginine-rich (SR) protein family have multiple functions in the pre-mRNA splicing reaction. In addition to being required for the removal of constitutively spliced introns, SR proteins can function to regulate alternative splicing both in vitro and in vivo (Ge & Manley, 1990; Krainer et al., 1990a; Fu et al., 1992; Zahler et al., 1993a; Caceres et al., 1994; Wang & Manley, 1995). In the cell, SR proteins migrate from speckles—subnuclear domains that may function as storage sites for certain splicing factors—to sites of active transcription (Misteli et al., 1997; Misteli & Spector, 1999) and some SR proteins have been found to shuttle in and out of the nucleus (Caceres et al., 1998). The subcellular localization of SR proteins can be modulated by phosphorylation (Misteli & Spector, 1998; Misteli et al., 1998) and this undoubtedly underlies some regulated splicing events. However, once in the nucleus and localized to the nascent pre-mRNA, exactly how SR proteins engage the general splicing machinery to recognize specific splice sites is unclear and is an area of intense investigation.