The composition of GEM is based on the results of experiments designed to determine the most supportive in vitro conditions for assisted reproduction and embryo manipulation.

GEM is also formulated to support sperm fertilizing ability. BSA supplementation (1 to 10 mg/ml) may be helpful during in vitro insemination, but sperm washing for in vivo inseminations can be carried out in GEM-PS or GEM-HEPES without additional protein supplementation.

This offers several important advantages over sperm washing media which contain protein, including markedly improved shelf-life and avoidance of both allergic reactions and risk of exposure to pathogens. Transfer of inseminated mouse ova into protein-free, equilibrated GEM, (under the conditions described above) is possible as early as 3 hours after co-culture with sperm.

GEM is a chemically-defined, protein-free, embryo culture medium.