*4.7. Clonogenic Assay*

Cells were seeded on 60 mm culture dishes at various densities and then treated with DMSO or KJ-28d (0.75 µM). After 2 h, cells were treated with the indicated doses of 137Cs γ-radiation. After 10 d, colonies were fixed and stained with 1.5% methylene blue (Sigma-Aldrich) in methanol solution. Colonies containing >50 cells were counted. DER was calculated as the dose (Gy) of radiation that yielded a surviving fraction of 0.1 for DMSO-treated cells divided by that dose for KJ-28d-treated cells. The experiment was performed in triplicate.
