For example, "Xenopus" embryos were used to identify the enzymes that control ubiquitination of Smad4, and to demonstrate direct links between TGF-beta superfamily signaling pathways and other important networks, such as the MAP kinase pathway and the Wnt pathway. Moreover, new methods using egg extracts revealed novel, important targets of the Wnt/GSK3 destruction complex. Cell division: "Xenopus" egg extracts have allowed the study of many complicated cellular events "in vitro". Because egg cytosol can support successive cycling between mitosis and interphase "in vitro", it has been critical to diverse studies of cell division. For example, the small GTPase Ran was first found to regulate interphase nuclear transport, but "Xenopus" egg extracts revealed the critical role of Ran GTPase in mitosis independent of its role in interphase nuclear transport. Similarly, the cell-free extracts were used to model nuclear envelope assembly from chromatin, revealing the function of RanGTPase in regulating nuclear envelope reassembly after mitosis. More recently, using "Xenopus" egg extracts, it was possible to demonstrate the mitosis-specific function of the nuclear lamin B in regulating spindle morphogenesis and to identify new proteins that mediate kinetochore attachment to microtubules. Embryonic development: "Xenopus" embryos are widely used in developmental biology. A summary of recent advances made by "Xenopus" research in recent years would include:   DNA replication: "Xenopus" cell-free extracts also support the synchronous assembly and the activation of origins of DNA replication. They have been instrumental in characterizing the biochemical function of the prereplicative complex, including MCM proteins. DNA damage response: Cell-free extracts have been instrumental to unravel the signaling pathways activated in response to DNA double-strand breaks (ATM), replication fork stalling (ATR) or DNA interstrand crosslinks (FA proteins and ATR). Notably, several mechanisms and components of these signal transduction pathways were first identified in "Xenopus". Apoptosis: "Xenopus" oocytes provide a tractable model for biochemical studies of apoptosis. Recently, oocytes were used recently to study the biochemical mechanisms of caspase-2 activation; importantly, this mechanism turns out to be conserved in mammals. Regenerative medicine: In recent years, tremendous interest in developmental biology has been stoked by the promise of regenerative medicine.