In the early days at Heiligenberg, Professor Hagen researched the radioprotective substances and cell death in lymphocytes and conducted research in Stockholm, in Professor Forssberg's laboratory. It was in Stockholm that Professor Hagen became acquainted with isolated DNA. There he realized the importance of the cellular effects of ionising radiation in relationship with DNA damage repair. Immediately following this, he began developing analytical techniques that would allow an examination and analysis of various types of DNA damage, which made Hagen an important pioneer in the field of Molecular Radiation Biology.