Abstract: A method for introducing a replication-defective retroviral vector into pluripotent stem cells of embryos of an avian species, including chickens, turkeys, quails or ducks. The method is useful for transferring nucleic acid sequences into embryonic avian cells which may differentiate into somatic or germ cells. Transfer into germ cells has been achieved to produce transgenic animals. The replication-defective retroviral vector used for transfer may be a recombinant retroviral vector containing both a retroviral derived nucleic acid sequence and a non-retroviral derived nucleic acid sequence. Examples of non-retroviral nucleic acid sequences are a neomycin resistance gene from the bacterial transposon Tn5, a herpes simplex virus thymidine kinase gene and a chicken growth hormone gene, however, any prokaryotic or eukaryotic nucleic acid sequence of interest may be used. Transgenic chickens have been produced whose cells contain and express a replication-defective retroviral vector nucleic acid sequence.