Three cell membrane receptor tyrosine kinases, Flt (also designated VEGF-R1),
Flk-1 (also designated VEGF-R2) and Flt-4, putatively involved in the growth of
endothelial cells, are characterized by the presence of seven immunoglobulinlike
sequences in their extracellular domain. These receptors exhibit high
degrees of sequence relatedness to each other as well as lesser degrees of
relatedness to the class III receptors including CSF-1/Fms, PDGR, SLFR/Kit
and Flt-3/Flk-2. Two members of this receptor class, Flt-1 and Flk-1, have
been shown to represent high affinity receptors for vascular endothelial
growth factors (VEGFs). On the basis of structural similarity to Flt and Flk-1,
it has been speculated that Flt-4 might represent a third receptor for either
VEGF or a VEGF-related ligand.