PTCH1 and Mobius syndrome: However, while irradiation of newborn Ptch1+/− mice with a CD1 background with a single dose of 3 Gy X-rays dramatically increased MBs over the spontaneous rate [9], no increase in MB incidence was observed in Ptch1+/− mice bred with a C57BL/6 background after neonatal irradiation [11].