Molecular mechanisms of renal hypertrophy: Role of p27^Kip1. There are two fundamentally different growth responses for cells comprising the nephron: hyperplasia or hypertrophy. Cells that progress through the normal cell cycle double their DNA content and eventually divide during mitosis. Those cells that hypertrophy stop the growth process in the G₁-phase of the cell cycle; while they increase in size, protein and RNA content, they cannot duplicate their set of chromosomes because they never pass through the S-phase of the cell cycle. Hypertrophy may be an early compensatory mechanism to initially replace the loss of functioning tissue, however, this maladaptive process eventually fosters progressive loss of renal function. Since progression of the cell through the G₁ to S-phases is regulated by cyclins D, E and A, which in turn bind and activate cyclin dependent kinases (CDKs), evidence has been accumulating on a particular CDK-inhibitor protein, p27^Kip1, which is speculated to be a key to the complex process of the G₁/S cell cycle transition. This article examines the mechanisms of the proliferative growth response following acute tubular necrosis, and compensatory hypertrophy of glomerular and tubule cells, with a particular focus on the protein p27^Kip1.