So far, we have seen that a regulated increase in MPF activity induces entry into mitosis. Presumably, the entry into mitosis is a consequence of the phosphorylation of specific proteins by the protein kinase activity of MPF. For the most part, however, the proteins phosphorylated by MPF in vivo have not been identified. The active phosphorylated forms of these proteins are thought to mediate the many remarkable events of mitosis including chromosome condensation, formation of the mitotic spindle, and breakdown of the nuclear envelope (see Figure 19-34).