In vitro fertilization (IVF) involves fertilization of mature oocytes with capacitated sperm in a tissue culture dish. This technique can generate large numbers of embryos with using only few males. These embryos can then be used in cryopreservation, injections, colony expanding and colony rederivation. IVF can also be used to generate offspring from cryopreserved sperm. The low IVF ability of frozen / thawed sperm can in some cases be overcome by zona melting. In addition, when a male mouse repeatedly fails to plug or impregnate normal female mice due to unknown reasons, IVF is one of the methods to overcome this problem.