Cyclin C was originally identified by a genetic screen for human and Drosophila cDNAs that complement a triple knock out of the CLN genes in Saccharomyces cerevisiae. Cyclin C is a nuclear protein present in a multiprotein complex. It interacts both in vitro and in vivo with cdk 8, a novel protein kinase of the cdk family. It is suggested that cyclin C and cdk 8 control RNA polymerase II function. Human cdk 8 cyclin C might be functionally associated with the mammalian transcription apparatus, perhaps involved in relaying growth regulatory signals.