The present invention includes alternatively and partially spliced cyclin C mRNAs, recombinant DNA and the truncated protein (a truncated cyclin C) they encode. The alternatively spliced mRNAs result from an insertion of unique exons containing premature termination codons. The partially spliced mRNAs result from an insertion of additional coding sequence derived from exons. One aspect of the present invention is the demonstration that at least one of the alternatively spliced cyclin C mRNAs is produced in a cell cycle dependent fashion, as is the novel truncated cyclin box protein that it encodes. Truncated cyclin C acts as an endogenously encoded cyclin C inhibitor by negatively regulating cyclin C/cdk8 complex activity, in much the same way as the cyclin dependent protein kinase inhibitors that inhibit the D-type cyclins, cyclin A and cyclin E.