Evidence is accumulating that directly responsible for the rate of progression of glomerular disease are specific positive (cyclins and cyclin-dependent kinases) and negative (cyclin-kinase inhibitors) cell cycle regulatory proteins. The challenge for nephrologists is to determine which ones are expressed in renal disease and their precise role in glomerular cell proliferation, hypertrophy and differentiation. Ultimately the goal is to find ever more appropriate therapeutic strategies to arrest or prevent progressive renal disease.