Retroviral Infection - Recombinant retroviruses produce virions, which are used to infect animal cells and mice embryos. When embryos beyond the 4-celled stage are used, often all the cells in an embryo may not be infected by the retrovirus, resulting in chimaeric mice.

A chimaera is an individual, which has in its body cells of two or more genotypes.The recombinant retrovirus RNA genome is copied by reverse transcriptase to yield a DNA copy (reverse transcription), which becomes integrated into the cell's genome.