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Here again, the cell cycle is a dramatic example. Of the three organisms most studied, Xenopus, fission yeast and budding yeast there appeared to be little in common. The frog embryonic cell cycle was controlled by a clock, the fission yeast by a rate controlling step in G2 and the budding yeast by a gene that initiated spindle pole duplication in G1. Yet all three activities turned out to encode the same cyclin dependent protein kinase, and the differences were only superficial.