CHEK1 and medulloblastoma: Olaparib inhibited parylation and induced CHK1 phosphorylation already 1 h after intraperitoneal (ip) injection, while accumulation of γH2AX was clearly detectable after 4 h, in P5 Ptch−/− cerebellar extracts (Fig. 7E), indicating that olaparib can cross the blood-brain barrier to reach medulloblastoma tissue.