AbstractA human fibrosarcoma cell line, HT‐1080‐6TG‐9AM, resistant to α‐amanitin at concentrations up to 10 μg/ml, was isolated after ethylmethanesulfonate mutagenesis and stepwise selection. The mutation is stable and dominant. RNA polymerase II purified from the mutant cells showed an altered affinity for labeled α‐amanitin and the sensitivity of the enzyme to the fungal toxin was decreased 50‐to 100‐fold. This functional test demonstrated that the biochemical basis for the resistance of the cells to α‐amanitin is due to an alteration o
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