Men whose fertility is threatened can elect to have semen frozen for later use. However, the preservation - and reactivation - of eggs, and tissue from the ovary, is far more problematic. In 2001 it was announced that frozen human ovarian tissue, once thawed, can grow in a "normal" fashion when implanted into mice, and immature follicles within the tissue, began to develop in a similar way to conventional ovarian tissue. Also, a new technique for cryopreservation of ovarian tissues appears to have greatly increased the number of frozen human eggs which can be subsequently fertilised. (Only 30 babies have been born worldwide using frozen eggs - the success rate is extremely low.) However, the restoration of working ovarian tissue, even allowing natural conception to take place, is still thought to be several years away.