Prepubertal boys treated for cancer are at risk of infertility as a result of their treatment. At present there are no options to preserve fertility in these boys. With informed consent, the investigators aim to cryopreserve (freeze) testis tissue biopsies taken from boys undergoing cancer therapy with a high risk of infertility. A portion of cryopreserved tissue will be securely stored, which in the future may be used to restore fertility in the patients. A second portion of the tissue will be used for laboratory work aimed at understanding the conditions required for development of the germ cells. Patients will be recruited from the Oncology Department at the Royal Hospital for Children and Young People (RHCYP) in Edinburgh. Tissue will be obtained from prepubertal patients with cancer who are about to undergo treatment with a high risk (\>80%) of infertility. Informed consent will be obtained from the patients legal guardian (and patient if applicable) for the procedure and subsequent use of tissue. A testis biopsy from one testis will be performed and if possible will coincide with a planned routine procedure requiring general anaesthetic in order to avoid the need for additional anaesthesia. The procedure will take place at RHCYP during the initial assessment and prior to the commencement of cancer treatment and will not result in delay of treatment. Tissue will be assessed for the presence of sperm for storage using existing methods. In addition, tissue will be cryopreserved by Tissue and Cells Directorate, SNBTS in Edinburgh. The tissue will be stored for up to 55 years in accordance with current legislation (HFEA Act as amended 2005). Research will be carried out at the MRC Centre for Reproductive Health in accordance with Human Tissue (Scotland) Act 2006.