We recently reported that the 'hydrophobic patch' (HP) of the Saccharomyces cerevisiae S-phase cyclin Clb5 facilitates its interaction with Orco (via its Cy or RXL motif), providing a mechanism that helps prevent rereplication from individual origins. This is the first finding of a biological function for an interaction between a cyclin and a cyclin-binding motif (Cy or RXL motif) in a target protein in Saccharomyces cerevisiae. It is also the first such example involving a B-type cyclin in any organism. Yet, some of our observations as well as work from other groups suggest that HP-RXL interactions are functionally important for cyclin-Cdk signaling to other targets. The evolutionary conservation of the HP motif suggests that it allows cyclins to carry out important and specialized functions. ©2005 Landes Bioscience.