ASCL1 and central nervous system cancer: OLIG2 is required for proliferation of multipotent neural progenitors and for glioma formation in a mouse model of gliomagenesis and it is also expressed in replicating oligodendrocyte precursor cells where it cooperates with ASCL1 to specify oligodendrocytes (Ligon et al., 2007; Parras et al., 2004).