The degree of spermatogenesis varies among infertile men from a barely compromised sperm number in the ejaculate to their complete absence-the condition known as azoospermia. The scarcity of gametes often represents a major hindrance to overcoming spermatogenic failure through the use of assisted fertilization techniques. This induced us to attempt replication of the spermatozoon by "male genome cloning." We then investigated whether it is possible to induce human spermatogenesis in host testes by transplanting germ cells obtained from human testicular biopsy specimens into mouse seminiferous tubules. After plating the sorted germ cells on a feeder layer, one putative spermatogonial stem cell colony proliferated up to day 9 of culture. The production of male gametes by ooplasmic somatic cell haploidization has been tested in our laboratory. Attempts to induce neogametogenesis from embryonic stem cells has yielded mouse offspring following fertilization with sperm-like cells.