Scott, on the other hand, spent nearly a year orbiting the Earth on a mission that started in March 2015. Most of the changes that Scott Kelly went through disappeared after hours or days of his return to Earth, but some changes stayed with him after six months.
About 7 percent of Scott's genes may have longer-term changes in expression after spaceflight, in areas such as DNA fix, the immune system, how bones are formed, hypoxia (an oxygen deficiency in the tissues), and hypercapnia (excessive carbon dioxide in the bloodstream). Scott Kelly set a US record with his a 340-day mission to the International Space Station.
NASA attributed this to "the stresses of space travel, which can cause changes in a cell's biological pathways and ejection of DNA and RNA". "However, a more pronounced decrease in speed and accuracy was reported postflight, possibly due to re-exposure and adjustment to Earth's gravity, and the busy schedule that enveloped Scott after his mission".
Scott Kelly (on the left), and Mikhail Korniyenko (on the right), were selected for the one-year mission in 2012. As identical twin astronauts, Scott and Mark were the ideal research subjects for the study's before-and-after comparison.
On his return, tests showed that parts of Scott's chromosomes, which contain DNA in which gene information is stored, had altered in shape. However, most of the telomeres returned to normal lengths within two days of Scott's return to Earth.
However, some changes persisted for six months.
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In particular, Chris Mason of Weill Cornell Medicine reported on the activation of Scott's "space genes" while confirming the results of his separate NASA study, published a year ago.
Findings show that about 93 percent of the changes in his genes were temporary and Kelly's genetic profile returned to normal after he came back to Earth.
Methylation, especially the locations near telomere regulation and collagen encoding gene regions, was documented in an epigenomics assessment, backing some of the other findings.
Kelly's one-year mission is a scientific stepping stone to a planned three-year mission to Mars, NASA said.
NASA is expected to release an integrated summary later this year, including the possible impact the findings will have on future space travel.
NASA originally released some of the basic findings previous year, but further reported details at the Investigators' Workshop in January.