The invention provides human protein complexes with endonuclease activity, including tRNA splicing endonuclease activity and 3′ end pre-mRNA endonuclease activity, and pre-ribosomal RNA cleavage activity. The invention also provides a splice variant of human Sen2, and human protein complexes comprising the variant. The invention provides human protein complexes with pre-ribosomal RNA cleavage activity. The invention also provides antibodies that immunospecifically bind to a complex described herein or a component thereof, and uses of such antibodies. The present invention also provides methods of preparing and purifying the complexes and uses of the complexes inter alia, in screening, diagnosis, and therapy. The present invention further provides methods of identifying a compound that modulates the expression of a component of a complex described herein, the formation of a complex described herein or the activity of a complex described herein, and methods of preventing, treating, managing or ameliorating a disorder, such as a proliferative disorder, or a symptom thereof utilizing such a compound.