In the longest animal space journey yet undertaken, three male C57BL/J10 mice flew aboard the Space Shuttle Discovery to the International Space Station and lived there for 91 days in a specially designed housing unit before returning to Earth on the Space Shuttle Atlantis. The Italian Space Agency funded and organized the project to gather information that could be used to reduce the health risks to humans during prolonged space missions.

Like other living organisms on Earth, humans are adapted structurally and functionally to the planet's gravitational field. Exposure to reduced gravity (or microgravity) in space thus affects almost all physiological systems, sometimes causing serious health problems that persist even after a return to normal gravity. These problems may become more pronounced during prolonged exposure to microgravity, as in interplanetary travel.

The systems most affected by microgravity include blood circulation, bone tissue and skeletal muscle and reproduction. Gene and protein expression are also thought to be altered. Data on these systems were collected from the astronaut mice and from control mice and then analyzed by 20 research groups from 6 countries.

Researchers headed by Angela Maria Rizzo (Università degli Studi di Milano, Italy) identified that damage to cells due to oxidative stress may be related to the reduction in red blood cell number and blood plasma volume known as 'space anemia' (PLoS One. 7, e32361; 2012). Sara Tavella (Università degli Studi di Genova, Italy) and her team reported tissue loss in weight-bearing bones, owing to both increased resorption and decreased deposition, that was partially prevented by overexpression of pleiotrophin ( PLoS One. 7, e33179; 2012). A collaborative effort headed by Romeo Betto (Institute of Neuroscience, Padova, Italy), Diana Conte (University of Bari, Italy) and Stefano Schiaffino (Venetian Institute of Molecular Medicine, Padova, Italy) found that slow-twitch muscle underwent microgravity-induced atrophy, whereas fast-twitch muscle seemed to resist atrophy (PLoS One. 7, e33232; 2012). Francesco Saverio Ambesi-Impiombato (Istituto Nazionale di Ricovero e Cura per Anziani, Rome, Italy) and colleagues noted structural and functional changes to the thyroid and dysfunctional spermatogenesis in the astronaut mice, suggesting that long-term exposure to microgravity could lead to reproductive dysfunction in male mammals (PLoS One. 7, e35418; 2012). And a group led by Yoshinobu Ohira (Osaka University, Japan) found that brain expression of many genes and proteins involved in a wide range of biological functions was influenced by exposure to microgravity (PLoS One 7, e40112; 2012).