Retroviral method: A retrovirus is a virus that carries its genetic material in the form of RNA rather than DNA. In this method retroviruses are used as vectors to transfer genetic material into the host cell, resulting in a chimera, an organism consisting of tissues or parts of diverse genetic constitution. Chimeras are inbred for as many as 20 generations until homozygous (carrying the desired transgene in every cell) transgenic offsprings are born. The method was successfully used in 1974 when a simian virus was inserted into mice embryos, resulting in mice carrying this DNA. In retroviral method, 8-cell stage embryo is removed and infected by virus carrying foreign gene. The retroviral is made defective in such a way that, it can effectively penetrate cells without replication.  The transformed embryo is then implanted back in foster mother. After birth, the matings are carried out to establish transgenic line. Although retrovirus is a good choice as a vector, it can carry only a limited size of the foreign DNA.