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When Mathai Mammen left a job overseeing R&D at Johnson & Johnson, he had his pick of what to work on next. During 25 years as a drug hunter — first co-founding Theravance Therapeutics, before joining the ranks of Merck & Co. and then J&J — he has advanced everything from small molecules to cell therapies into settings from infectious diseases to oncology. In joining FogPharma, he has backed a potentially transformative but still unproven modality whose time may have at last arrived: cell-penetrating helical peptides.