Three receptor tyrosine kinases, FLT1, FLK1 and FLT4, contain seven immunoglobin-like domains in their extracellular region and are strongly related by sequence similarities to each other and, to a lesser degree, to the class III receptors CSF1R/FMS, PDGFR, SLFR/KIT and FLT3/FLK2. They constitute a family of receptors putatively involved in the growth regulation of endothelial cells. We describe here the structure and pattern of expression of the human FLT4 gene. Two FLT4 transcripts of 5.8 and 4.5 kb are expressed in the human placenta and several hematopoietic cell lines. In mouse, a 5.8-kb transcript is expressed in a variety of tissues. A translational product 1298 amino acids in length is predicted to be encoded by the largest open reading frame. The FLT4 protein, when transiently expressed in Cos-7 cells and immunoprecipitated with a FLT4-specific rabbit immune serum, has an apparent molecular weight of 170 kDa.