PTCH1 and neoplasm: In a mouse SHH-MB conditional model in which Ptch1 is homozygously deleted at embryonic day 14 (E14) in all GCPs (Atoh1-Cre), additional homozygous loss of Pten leads to rapid tumor formation, and heterozygous loss leads to a slightly more aggressive phenotype than Ptch1 loss alone13.