The fact that animal introns are not spliced out in plants suggests that (1,2).Among the few well-characterized metazoan splicing factors are the proteins SF2/ASF, which is involved in selection of the 5' splice site, and U2AF, which stimulates the interaction of U2 snRNP with the branch point region. It has been shown that these and other splicing factors contain a similar structural motif, termed the RS domain (3). These proteins are phosphorylated in vivo and share a serine phosphoepitope recognized by monoclonal antibody mAbl04 (4, 5). They were termed SR proteins and comprise a family of evolutionary conserved nuclear phosphoproteins, which contain either one or two RNA recognition motifs and sequences of alternating serine and arginine (SR) residues (3,5