He was among the first German radiation biologists to become aware of the important role of DNA repair in endpoints such as mutagenesis, aberration formation, cell death, and carcinogenesis.

Ulrich Hagen graduated from the Ludwig Maximilian University of Munich with doctoral degrees in Medicine and Biology.

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.

In 1965, he accepted an appointment at the Institute of Radiation Biology of the Kernforschungszentrum Karlsruhe (under the leadership of Karl Günter Zimmer).

During those years, he recognized and documented the problem of RNA polymerase inhibition by damaged DNA templates and was among the first German radiation biologists to become aware of the important role of DNA repair, as relates to endpoints: mutagenesis, aberration formation, cell death, and carcinogenesis.