Abstract: The present invention belongs to the field of biotechnology. More specifically, the invention relates to chimeric fusion proteins comprising a light activated protein domain, e.g., a newly characterized light-oxygen-voltage-sensing (LOV) domain or a light sensing domain of the cyanobacterial phytochrome (PHY) CPH1, wherein the chimeric fusion protein is capable of dimerizing upon excitation with light of a suitable wavelength. Said fusion proteins further comprise the intracellular part of a receptor tyrosine kinase (RTK). The invention further relates to nucleic acid molecules encoding said chimeric fusion proteins; non-human transgenic animals expressing the chimeric fusion protein encoded by said nucleic acid molecules; as well as uses of said chimeric fusion proteins, e.g. in a screening method.