The majority of G0 transgenic mice are derived from mosaic embryos

Most transgenic mice are generated by the direct microinjection of DNA fragments into the pronuclei of fertilized eggs. It has been generally assumed that the majority of integration events occur prior to the first round of chromosomal DNA replication (Palmiter and Brinster, 1986). In this study we have determined by comparison of PCR, Southern blot and transmission frequencies that at least 62% of integration events generate a mosaic (somatic and/or germline) G₀ transgenic mouse. Furthermore, the statistical probability of transgene-containing cells segregating to the various early embryo lineages implies that this is probably an underestimate of the true mosaic frequency. Thus, the majority of DNA injected into fertilized mouse eggs integrates after the first round of chromosomal DNA replication, therefore most G₀ transgenic mice are derived from a mosaic embryo.