Six independent p53+/GFP clones were injected into blastocysts and transferred to pseudo-pregnant females using standard procedures. Strikingly, no pregnancies were obtained. It has been shown that the p53 pathway is regulated very differently in ES and somatic cells: ES cells contain relatively high p53 levels and lack the p53-mediated DNA damage responses found in somatic cells (18). This, together with the observation that p53 levels decrease during mouse embryogenesis (19), suggested an explanation for the observed lack of pregnancies: we speculate that the high levels of p53GFP in the ES cells injected into blastocysts might have prevented normal embryonic development once these cells began to differentiate and the p53 pathway became functional.