Title: Genetically modified animals, in particular mice, are unique mutants with a major scientific potential. Cryopreservation of pre-implantation embryos or of spermatozoa is a common approach to protect those lines against loss or to archive them. Prerequisite to take a mutant line out of a breeding nucleus is the cryopreservation of a sufficient number of samples and a powerful quality assessment.Background: Several common protocols to cryopreserve spermatozoa and subsequently to forward these to an in vitro-fertilization under standardized conditions are published using donors of a strictly limited age-span. In practice, several limitations happen frequently for those approaches, e.g. the availability of genetically modified males, in particular if a line is in an experimental use or the mutation leads to restrictions. Female donors are often received from external sources and are subsequently not available before weaning. They are subject of several delays to induce an early superovulation. The aim of this report is to study the suitable age span of sperm and oocyte donors.