LIGANDS FOR flt3 RECEPTORS CAPABLE OF TRANSDUCING SELF-RENEWAL SIGNALS TO REGULATE THE GROWTH, PROLIFERATION OR DIFFERENTIATION OF PROGENITOR CELLS AND STEM CELLS ARE DISCLOSED. THE INVENTION IS DIRECTED TO flt3-L AS AN ISOLATED PROTEIN, THE DNA ENCODING THE flt3-L, HOST CELLS TRANSFECTED WITH cDNAs ENCODING flt3-L, COMPOSITIONS COMPRISING flt3-L, METHODS OF IMPROVING GENE TRANSFER TO A MAMMAL USING flt3-L, AND METHODS OF IMPROVING TRANSPLANTATIONS USING flt3-L. Flt3-L FINDS USE IN TREATING PATIENTS WITH ANEMIA, AIDS AND VARIOUS CANCERS.