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Figure S3. Addition of the Aurora B inhibitor hesperadin blocks spindle checkpoint signaling in X. laevis extracts. The spindle checkpoint was activated in M phase extracts by the addition of 10,000 sperm per microliter and nocodazole. CSF was destroyed by the addition of sperm and Cdk1 activity was assayed to assess spindle checkpoint signaling. In DMSO controls, the checkpoint is activated and histone H1 kinase activity remains high. Hesperadin addition inhibits checkpoint signaling and H1 kinase activity is decreased.