"This is a Ph.D. dissertation. This thesis deals with the eukaryotic protein NIPP1 (Nuclear Inhibitor of Protein Phosphate-1), which was initially described by the host laboratory in 1992 and still is the major focus of its research efforts. The first part of the introduction describes our current insights into the structure and function of NIPP1, with the exception of aspects that directly deal with this doctoral project. For reasons of clarity, the data are presented in a logical manner, which implies that they are not necessarily presented according to their order of discovery. In the final sections of the introduction, a brief overview will be given of the state of the art of the `structure and function of ribonucleases' and `transcriptional repression mechanisms,' two topics that directly related to this research project. Contents include: Introduction, The Structure and Function of NIPP1, The Structure and Function of Ribonucleases, Transcriptional Repression in Eukaryotes, Aims and Strategies, Materials and Methods, Results and Discussions, Mapping of the RNA-binding and Endoribonuclease Domain of NIPP1, Screening for Proteins that Interact with the C-Terminal domain of NIPP1."