ROCK1 and ROCK2 help cells to divide by enabling a process called actomyosin contractility (left). This process also promotes the production of three proteins (CDK1, CyclinA and CKS1) that have an important role in causing cells to increase in number (proliferate). These proteins drive the cell cycle that underpins the proliferation of both normal and cancerous cells: DNA replication occurs in stage S of the cycle, with cell division taking place in stage G2/M. Kümper et al. found that the loss of both ROCK genes, or the use of drugs that inhibit ROCK activity, can permanently stop mouse cells from dividing (right). This phenomenon, which is known as cellular senescence, is probably caused by the loss of actomyosin contractility, which leads to lower levels of CDK1, CyclinA and CKS1.