Frozen ovaries reduce the number of laboratory animals needed 

Many different genetically modified mouse strains have been produced since the 1980s, when scientists discovered a way to produce extra human proteins by introducing extra genes into mouse embryos. These strains provide insight into the working of genes, and in some cases act as suitable models for human diseases to aid in the development of new preventative, diagnostic and therapeutic medical approaches. However, this has led to such an increase in the number of mice that laboratories have begun to run out of space to house them. From an ethical standpoint it is better to preserve these unique strains by freezing their embryos or sperm, but freezing embryos is very wasteful of animals and mouse sperm is so prone to damage that scientists must exhaustively test the quality of frozen sperm before use, which in turn also costs the lives of many animals used as egg cell donors. 

In order to reduce the number of animals needed for testing and the suffering inflicted on embryo and egg cell donors, researchers have begun experiments and tests of freezing mouse ovaries instead. Frozen ovaries from the 'archive' can be transplanted into a receiver mouse from the same strain. When this receiver mates with a normal male, she can give birth to young with genetic mutations. The host is unlikely to reject the transplant, as all of the animals within a certain strain are very closely related. The popular Black 6 strain, whose sperm is generally not suitable for freezing, was the strain that responded the best to this process. 

This technique offers a safe solution for the preservation of unique mouse strains while reducing the number of animals necessary. It therefore offers an alternative for animal testing using the 3-R principle of Reducing, Refining and Replacing animal tests.