Thanks to chemo- and radiotherapy, the life expectancy for children with cancer has increased considerably. However, these cancer treatments have a serious side effect: sterility. The banking of semen is offered to adult men to circumvent this side effect. Nevertheless, prepubertal boys can not benefit from this option, because spermatogenesis has not been initiated yet. A possible solution to preserve their fertility is banking and transplanting spermatogonial stem cells. Before spermatogonial stem cell transplantation (SSCT) can be offered, the safety of the technique has to be investigated. Therefore, both the genetic and epigenetic characteristics of spermatozoa and offspring, obtained by SSCT are studied in a mouse model.