The invention relates to modulating the SIRPα - CD47 interaction in order to increase hematopoietic stem cell engraftment and compounds therefor. In some embodiments, there is provided isolated SIRPα and CD47 polypeptides, fragments and fusion proteins for enhancing hematopoietic stem cell engraftment. Further there is provided methods for increasing hematopoietic stem cell engraftment through administration of the above polypeptides.