LRFN4 is one of a family of five transmembrane glycoproteins that are highly expressed in neuronal tissues. LRFN proteins share leucine-rich repeat (LRR)-immunoglobulin-like (Ig)-fibronectin type III (Fn)-transmembrane domain structure with other members of the LRR-Ig-Fn protein superfamily such as the Slitrk family of proteins. Expression of LRFN1, -3, and -4 mRNA was detected in embryonic neuronal cells, while Lrfn2 and Lrfn5 expression was primarily restricted to more mature cells. LRFN1, -2, and -4 bound to PDZ domains of postsynaptic PSD95, re-distributing PSD95 to the cell periphery. It has been suggested that the Lrfn proteins play a role in the developing and/or mature vertebrate nervous system. At least two isoforms of LRFN4 are known to exist.