PTCH1 and medulloblastoma: However, since the reduction of Dnmt1 activity in heterozygous Ptch mice did not affect initiation of medulloblastoma [11] and since reduced levels of Dnmt1 activity can inhibit proliferation and induce apoptosis [43], it is possible that the already initiated medulloblastoma simply underwent pronounced apoptosis and thus stagnated in their growth.