Soyuz TM-21 was Soyuz mission, a human spaceflight mission transporting personnel to the Russian space station Mir Part of the US/Russian Shuttle-Mir Programme, the mission launched from Baikonur Cosmodrome, atop a Soyuz U2 carrier dock, at 06:11:34 UTC on March 14, 1995. It is of note because its launch marked the presence, for the first time ever, of thirteen humans in space simultaneously - three aboard the Soyuz, three aboard Mir and seven aboard Space Shuttle Endeavour, flying STS-67.

The spacecraft carried expedition EO-18 to the space station, including the first American astronaut to launch on a Soyuz spacecraft and board Mir, Norman Thagard, for the American Thagard Increment aboard the station, the first Increment of the Shuttle-Mir program. The three crew members it launched were relieved by Space Shuttle Atlantis during STS-71, when they were replaced by expedition EO-19, who returned to earth aboard Soyuz TM-21 on September 11, 1995.

The metabolic research on STS-71 included studies of the cardiovascular and pulmonary systems; neurosensory research; hygiene, sanitation, and radiation research; behavior and performance research; fundamental biology research; and microgravity research.
Deconditioning of the cardiovascular and pulmonary system, with the occurrence of orthostatic intolerance (or lightheadedness upon standing) observed in returning crew members of long duration spaceflight is of primary interest to researchers. Researchers measured changes in blood volume during flight and the pooling of blood in the legs and abdomen upon reentry.
Investigations in neurosensory research begun during Mir-18 focused on the mixed messages the body receives when the brain integrates nerve impulses from the eyes, inner ear, muscles and joints. The brain can no longer rely on gravity as a constant in determining body position and orientation. Two of these studies seek to enhance our understanding of how humans adapt to spaceflight and readapt to Earth's environment.
Two investigations in hygiene, sanitation, and radiation research looked at the radiation environment experienced during an extended stay in space, and two others looked at the presence of microbes or trace chemicals found in the air and water consumed by the astronauts.
Data from tests to study the long-term effects of microgravity on muscle coordination and mental acuity, collected during more than three months on Mir-18, were returned to Earth aboard Atlantis. A Russian spacecraft control simulator used before, during and after flight, allowed researchers to measure crew member's functional state and manual control performance.
STS-71 studied how weightlessness affects embryo development by returning to Earth a set of pre-fertilized quail eggs incubated on board Mir. The incubation process was stopped at various stages of development, and the embryos placed in a fixative solution for later analysis. Improved sensors were carried to orbit by Atlantis to be added to the Mir station greenhouse by the Mir-19 crew. The updated greenhouse then was ready for plant experiments on future NASA-Mir missions.
STS-71 carried several hundred protein samples, frozen in a thermos-bottle-like vacuum jacket or dewar, to the Mir space stationAfter they thaw, the proteins crystallized until the dewar is retrieved by the STS-74 crew in November 1995.
The months-long growing time aboard Mir should produce large protein crystals of sufficient size and quality to compare with corresponding crystals grown in Earth-based laboratories. The Mir experiment was used to evaluate the effectiveness of flash-frozen bath and liquid-liquid diffusion techniques for growing protein and virus crystals on long-duration space missions. Experience gained will help shape PCG investigations to be conducted aboard the International Space Station.