Hek ligand (hek-L) polypeptides as well as DNA sequences, vectors and transformed host cells useful in providing hek-L polypeptides. The hek-L polypeptides bind to a cell surface receptor (hek) that is a member of the receptor tyrosine kinase family. Hek is expressed on cells that include certain tumor cell lines. The hek-L polypeptides also bind a distinct receptor tyrosine kinase known elk.