[en] The authors are using MOCA14 track structure code of Wilson and Paretzke to generate detailed distributions of energy deposition by protons, α-particles and X-rays in cylindrical target volumes of dimensions 1-50 nm, including those corresponding to known DNA structures. Comparison of these with observed RBEs of α-particles of different energies suggests that a critical property may be deposition of threshold energy of --300 eV in a nucleosome sized target. Speculatively hypothesizing, then, that nucleosomes are the critical targets for densely ionizing radiations, leads to the conclusion that the integrity of 2-4% of the cellular genome is critical for cell survival. This interpretation may be consistent with the observed effectiveness of DNA-incorporated Iodine-125