We describe an ELABELA polypeptide comprising a sequence CXXXRCXXXHSRVPFP (SEQ ID NO: 1), in which X signifies an amino acid residue, such as a sequence selected from the group consisting of SEQ ID NO: 2 to SEQ ID NO: 18, preferably CLQRRCMPLHSRVPFP (SEQ ID NO: 2), or a fragment, homologue, variant or derivative thereof, which polypeptide is capable of maintaining self-renewal and/or pluripotency of a stem cell.