Zirkle (in 1970) was one of the first researchers to observe that, when just one chromosome is retarded to arrive at the metaphase plate, anaphase onset is postponed until some minutes after its arrival. This observation, together with similar ones, suggested that a control mechanism exists at the metaphase-to-anaphase transition. Using drugs such as nocodazole and colchicine, the mitotic spindle disassembles and the cell cycle is blocked at the metaphase-to-anaphase transition. Using these drugs (see the review from Rieder and Palazzo in 1992), the putative control mechanism was named Spindle Assembly Checkpoint (SAC). This regulatory mechanism has been intensively studied since.