To identify receptor tyrosine kinases (RTKs) critical to early hematopoiesis, we performed polymerase chain reaction-based cloning from yolk sac and highly enriched bone marrow hematopoietic stem cells (HSCs). Characterization of two novel genes of their full-length cDNA sequences revealed that they were mouse homologues of the endothelial cell RTK genes, TIE and TEK. They shared a unique structural property of coexistent immunoglobulin-like domain, epidermal growth factor-like repeats, and fibronectin type III repeats in their extracellular domains. Both genes were expressed in a similar fashion in adult tissues and primitive hematopoietic cells, predominantly in the bone marrow HSCs.