What happens during tumor remission? After chemotherapy, tumor cell numbers may be reduced, or surviving cells may enter an altered, dormant state. Evidence for dormancy now comes from Catherine Shachaf, Dean Felsher (Stanford University, Stanford, CA), and colleagues. In their mouse model, liver tumors can be forced into dormancy and then reawakened at will.

As in previous studies, regression was induced by shutting off the oversupply of MYC, in this case using a tetracycline-controlled promoter. Transgenic mice took ~12 weeks to develop the MYC-induced tumors, but within 4 days of MYC inactivation the cells died or differentiated into normal liver cells. Tumors regressed but could later be reactivated by switching MYC back on.

The persistence of dormant tumor cells was demonstrated by transplanting tumor cells containing transgenes for both luciferase and the tetracycline-controlled MYC. Even when MYC was inactivated the luciferase signal persisted in the...

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