Appears to regulate cell differentiation within the neural tube. May regulate the differentiation of cell types along the dorsoventral axis of the neural tube, acting in conjunction with distinct ventralizing signals from the notochord and floor plate. Controls the cell differentiation in the neural tube in several ways: (1) promotes the differentiation of cell types that derive from the dorsal neural tube. (2) ensures that the dorsal neural tube is refractory to ventralizing species from the notochord. (3) can diffuse and influence the fate of cells in more ventral regions of the neural tube