PTCH1 and leukemia: In addition to the Cxcl3 chemokine, the genes down-regulated in Ptch1+/−/Tis21KO GCPs include: Timp1, which is involved in the migration of human neural stem cells as well as leukemia cells by activating PI3K (57, 58); Sik2, necessary to the G1/S transition and activator of PI3K (59); and the tumor suppressor Smg1, which antagonizes mTOR by negatively modulating its overactivation, since Smg1 deletion leads to hyper-proliferation (60).