Cyclin dependent kinases are themselves catalytically inactive due to the fact that their active site is blocked by a portion of the Cdk molecule itself. Binding to their corresponding cyclin partner results in conformational change that partially exposes the active site. The two B-type cyclins localize to different regions within the cell and and are thought to have specific roles as CDK1-activating subunits (see Bellanger et al., 2007). Cyclin B1 is primarily cytoplasmic during interphase and translocates into the nucleus at the onset of mitosis (Jackman et al., 1995; Hagting et al., 1999). Cyclin B2 colocalizes with the Golgi apparatus and contributes to its fragmentation during mitosis (Jackman et al., 1995; Draviam et al., 2001).