Cyclin-dependent kinases are essential activators of cell cycle progression. Recent work, largely in yeast, has shown that Cdks not only promote cell cycle transitions but also function to regulate gene expression and other cellular processes. The Pho85 Cdk was originally identified because of its role in regulating acid phosphatase gene expression but strains lacking PHO85 show a wide spectrum of phenotypes, indicating many cellular functions for Pho85. Consistent with the multifunctional nature of Pho85, we and others have identified 10 genes encoding Pho85 cyclins or Pcls. We are studying the Pho85 family of Cdks both as a genetic system with which to dissect the basis of Cdk specificity and as a superb model for analysing the role of Cdks in multiple cellular functions. Synthetic dosage lethality (SDL) screens in our laboratory have also been successful in identifying Pho85 substrates and we have now extended this approach to look for substrates for all budding yeast kinases.