Microgravity or an altered gravity environment from the static 1g has been shown to influence global gene expression patterns and protein levels in cultured cells or animals, but it is unclear how these changes in gene and protein expressions are related to each other or are related to other factors regulating such changes. Recent advancement in the field of molecular biology revealed that a different class of RNA, the small non-coding microRNA (miRNA), can have a broad effect on gene expression networks by mainly inhibiting the translational process. In this experiment conducted on the International Space Station, we propose to test the hypotheses that miRNA profiles will be altered in the space environment, and that cellular responses to DNA damage in space are different from those on the ground.