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The largest subunit of the RNA polymerase II (pol II) contains at the carboxy-terminus a peculiar repetitive sequence that consists of 52 tandem repeats of the consensus motif Tyr-Ser-Pro-Thr-Ser-Pro-Ser, referred to as the C-terminal domain (CTD). Upon transcriptional initiation/promoter clearance, the CTD becomes extensively phosphorylated and apparently remains so during elongation. While the underphosphorylated CTD plays a role in transcriptional initiation, recent evidence couples the highly phosphorylated CTD to RNA processing, namely polyadenylation and splicing. Using a yeast two-hybrid screen, we have selected for human proteins that interact with the CTD of RNA polymerase II. The CTD-GAL fusion protein used as a bait is highly phosphorylated in yeast and, accordingly, we did not isolate proteins implicated in transcriptional regulation but rather proteins with possible roles in RNA splicing. One major cDNA clone isolated this way encodes SRrp129/CASP11, a protein that contains a conserved CTD-interaction domain at the C-terminus and an internal serine-arginine rich domain (SR domain). Proteins of the SR family have been implicated in RNA splicing, notably in the regulation of alternative splicing. Thus we consider it likely that SRrp129 is an auxiliary splice factor. We also improved our method to quickly map domains involved in protein-protein interaction (Stagljar et al., 1996, BioTechniques 21, 430-432). Instead of using sonication for the production of a random DNA fragment library, we took advantage of the fact that DNAse I in the presence of manganese (II) produces double strand rather than single strand DNA breaks. The DNA fragment library of the SRrp129 clone was then used in the yeast two-hybrid system to identify the 100-amino acid domain that interacts with the CTD of RNA polymerase II.