Female sperm is a term that traitionally refers to a sperm that contains an X chromosome (since when such a sperm fertilizes an egg, a female daughter is born) produced, in the usual way by a male. However, for over 20 years, dating back to the late 1980s, scientists have explored how to produce sperm where all of the chromosomes come from an adult woman. In the late 1990s, theory become a partial reality when scientists in Japan developed chicken female sperm, by injecting bone marrow stem cells from a female chicken into a male chicken's testicles. This technique has not yet been successfully adapted for use by humans.