Signaling adapter that couples activated growth factor receptors to signaling pathway in neurons. Involved in the signal transduction pathways of neurotrophin-activated Trk receptors in cortical neurons. Interacts with the Trk receptors in a phosphotyrosine-dependent manner and MEGF12. Once activated, binds to GRB2 (By similarity). Expressed in brain. Expressed at high level in the hypothalamus and at low level in the caudate nucleus. The PID domain mediates binding to the TrkA receptor. Phosphorylated on tyrosine by the Trk receptors.