Exencephalia: its occurrence in untreated mice

Exencephalia has been reported in irradiated CFI mice but there are no reports of its occurrence in untreated mice of this strain. In the course of establishing disease-free breeding colonies from CFI female mice delivered of their offspring by cesarean section, exencephalia was seen frequently. During a 2-week period, 90 litters were delivered; 11 contained exencephalic fetuses, at the rate of one per litter (11 of 90 litters, 12.2 percent; 11 of 1056 fetuses, 1.04 percent). The prevalence of this anomaly in untreated mice of this strain could contribute to overestimates of the effectiveness of low doses of radiation.