Olig2, also known as Oligo2, is a basic helix-loop-helix transcription factor which is expressed in oligodendroglial tumors of the brain. Oligo2 is structurally similar to OLIG1. Both of them have non-overlapping developmental functions in patterning, neuron subtype specification and the formation of oligodendrocytes. Olig2 is required for oligodendrocyte and motor neuron specification in the spinal cord, as well as for the development of somatic motor neurons in the hindbrain. Its over-expression inhibits neural progenitor proliferation through changes in potassium channel activity, thereby contributing to the reduced neuronal numbers and brain size in down syndrome. Olig2 may be a cause of a form of T-cell acute lymphoblastic leukemia.