 Working in Xenopus (frog) egg extracts, Kirschner and Andrew Murray showed that cyclin synthesis drives the cell cycle  and, later, that ubiquitin regulates levels of cyclin by marking the cell-cycle molecule for destruction.His lab discovered and purified many of the components involved in cell cycle progression, including anaphase promoting complex (APC), the complex that ubiquitinates cyclin B.A second noted finding was his discovery, with Tim Mitchison, of the dynamic instability of microtubules, In mitosis, for example, microtubules form the spindle that separates the chromosomes.