 In contrast to FGFRs 1-4, it lacks a cytoplasmic tyrosine kinase domain and one isoform, FGFR5γ, and only contains the extracellular domains D1 and D2.The FGFRs are known to dimerize as heterodimers and homodimers.So far, five distinct membrane FGFR have been identified in vertebrates and all of them belong to the tyrosine kinase superfamily (FGFR1 to FGFR4).