KRIT1 and cerebral cavernous malformation: However, CCMs are mainly restricted to the hindbrain and retina, where angiogenesis persists until P10, and tamoxifen treatment after P10 does not result in CCM lesion formation, suggesting that homozygous loss-of-function of KRIT1 only predisposes to CCM pathogenesis, which requires the additive effect of microenvironmental determinants, including local pro-angiogenic conditions, and is influenced by genetic modifiers [7,8,91].