The human FLT3 cDNA was cloned from a pre-B cell line and characterized. The deduced amino acid sequence shows that FLT3 codes for a receptor-type tyrosine kinase of 993 residues, presenting a strong similarity with the corresponding mouse FLT3/FLK2 protein as well as with the receptors for colony-stimulating factor 1 (CSF1R/FMS) and steel locus factor (SLFR/KIT). An analysis of the expression of the gene using amplification of reverse transcribed FLT3 mRNA by polymerase chain reaction shows that FLT3 is expressed in various lymphohematopoietic cells and tissues, including a series of immature cell lines and leukemias of lymphocytic origin.