Human-specific protein that promotes neural progenitor proliferation and evolutionary expansion of the brain neocortex by regulating the Notch signaling pathway. Able to promote neural progenitor self-renewal, possibly by down-regulating neuronal differentiation genes, thereby delaying the differentiation of neuronal progenitors and leading to an overall final increase in neuronal production. Acts by enhancing the Notch signaling pathway via two different mechanisms that probably work in parallel to reach the same effect. Enhances Notch signaling pathway in a non-cell-autonomous manner via direct interaction with NOTCH2. Also promotes Notch signaling pathway in a cell-autonomous manner through inhibition of cis DLL1-NOTCH2 interactions, which promotes neuronal differentiation.