The fact that X-irradiation of CF1 and ICR female zygotes did neither lead to an increase of developmental defects nor to an increase of genomic instability in the next generation contrasts with the positive results obtained by Pils et al. [10] after Xirradiation of Heiligenberger female zygotes. Our negative findings regarding these two endpoints could be due to the use of much lower doses (0.2 and 0.4 Gy instead of 1 Gy), but also to differences in strain radiation sensitivities. Of all investigated mouse strains, the Heiligenberger zygote revealed to be clearly the most sensitive to radiation-induction of external malformations [1–4]. As far as genomic instability is concerned, studies by Ullrich and co-workers have also shown that considerable differences in sensitivity exist between mouse strains [29].