When bound to ligand, the kinase is activated and this activation can signal many downstream events in the cell, including activation of new gene transcription. ELF-1 is expressed by neurons in a graded fashion across the tectum, with highest levels posteriorly and lowest levels anteriorly. The RTK receptor for ELF1 is called MEK4 and it is expressed by retinal ganglion cells and their axons. Again there is a gradient, with highest levels of MEK4 expressed by temporal retinal cells and lowest levels by nasal retinal cells. Temporal axons would be the most sensitive to the ELF1 gradient because they have the highest level of the receptor.
Evidence is available in the case of the tectum, and it points to a homeobox gene, engrailed.
This gene is a vertebrate homologue of a Drosophila segment-polarity gene of the same name. In vertebrates, engrailed is itself expressed in the very early developing tectum. Expression is present in a gradient oriented the same way as the later ELF1 gradient, with highest levels posteriorly and lowest levels anteriorly. Engrailed is expressed earlier than ELF1, and hence might be a transcriptional activator of ELF1. Direct evidence for an upstream role of engrailed was obtained by using a virus carrying the engrailed gene to infect anterior tectal cells.