rapid. Once in the nucleus, cyclin B1/CDK1 phosphorylates many targets in preparation for mitosis, including histone H1, nuclear lamins, centrosomal proteins, and microtubule associated proteins (MAPs). The subcellular localization of cdc25 also shifts from the cytosol to the nucleus during prophase. This is accomplished via removal of nuclear localization sequence (NLS)-obscuring phosphates and phosphorylation of the nuclear export signal. It is thought that the simultaneous transport of cdc25 and cyclin-B1/CDK1 into the nucleus amplify the switch-like nature of the transition by increasing the effective concentrations of the proteins. G2/M DNA damage arrest Cells respond to DNA damage or incompletely replicated chromosomes in G2 phase by delaying the G2/M transition so as to prevent attempts to segregate damaged chromosomes. DNA damage is detected by the kinases ATM and ATR, which activate Chk1, an inhibitory kinase of Cdc25. Chk1 inhibits Cdc25 activity both directly and by promoting its exclusion from the nucleus. The net effect is an increase in the threshold of cyclin B1 required to initiate the hysteretic transition to M-phase, effectively stalling the cell in G2 until the damage is repaired by mechanisms such as homology-directed repair (see above). Long-term maintenance of the G2 arrest is also mediated by p53, which is stabilized in response to DNA damage. CDK1 is directly inhibited by three transcriptional targets of p53: p21, Gadd45, and 14-3-3σ. Inactive Cyclin B1/CDK1 is sequestered in the nucleus by p21, while active Cyclin B1/CDK1 complexes are sequestered in the cytoplasm by 14-3-3σ. Gadd45 disrupts the binding of Cyclin B1 and CDK1 through direct interaction with CDK1. P53 also directly transcriptionally represses CDK1. Medical relevance Mutations in several genes involved in the G2/M transition are implicated in many cancers. Overexpression of both cyclin B and CDK1, oftentimes downstream of loss of tumor suppressors such as p53, can cause an increase in cell proliferation. Experimental approaches to mitigate these changes include both pharmacological inhibition of CDK1 and downregulation of cyclin B1 expression (e.g., via siRNA). Other attempts to modulate the G2/M transition for chemotherapy applications have focused on the DNA damage checkpoint. Pharmacologically bypassing the G2/M checkpoint via inhibition of Chk1 has been shown to enhance cytotoxicity of other chemotherapy drugs. Bypassing the checkpoint leads to the rapid accumulation of deleterious mutations, which is thought to drive the cancerous cells into apoptosis. Conversely, attempts to prolong the G2/M arrest have also been shown to enhance the cytotoxicity of drugs like