Are biological effects of space radiation really altered under the microgravity environment?

•Two major factors of space environment are space radiation and microgravity.•The interactive biological-effects of these two factors remain not well understood.•We propose a new concept to solve this problem, for example, gene expression analysis.

Two major factors of space environment are space radiation and microgravity. It is generally considered that a high level of ionizing radiation (IR) in space has an influence on living organisms including humans; therefore, the possible alteration of space-radiation influences by the microgravity environment is of great concern. In fact, examination of such a possibility has been extensively conducted since the early days of space experiments, suggesting a possible synergistic effect of radiation and microgravity in some experiments but a negative observation in others. Because these complicated results remain not well understood, we propose a solution to this problem. Gene expression analysis is one of the solutions to the problem. In fact, gene expression may be changed by microgravity, and further modification may be possible through IR. This result could reveal an interactive effect of both factors on the cellular responses, which could in turn reveal whether the human-health abnormalities expected under the microgravity environment can be altered by space radiation. We believe that this is a new aspect in the study of the interactive effect of radiation and microgravity. However, further improvements in space experimental technologies are required for future studies.