Non-naturally occurring polypeptide (I) is claimed, whose aminoacid sequence is similar enough to naturally-occurring stem cell factor (SCF) to give it the same haematopoietic biological activity. (I) is produced by eukaryotic or prokaryotic expression of an exogeneous DNA sequence, and has in vivo and in vitro biological activity. Also claimed are a DNA sequence encoding (I), a host transformed with the DNA sequence, a process for producing and recoverng SCF, a DNA sequence encoding a human SCF analogue, an antibody that binds SCF, a compsn.; of (I) with a water-soluble polymer, a method of preparing the above compsn. and a method of transfecting early haematopoietic stem cells. Routes of administration are e.g. parenteral, pulmonary, nasal and oral.