A polypeptide may include an amino acid sequence having at least about 95% identity to the amino acid sequence of SEQ ID NO: 42, wherein a residue at position 94 of the polypeptide is an amino acid other than threonine or wherein a residue at position 82 of the polypeptide is an amino acid other than serine. A polypeptide may include at least the first 14 residues and at most the first 92 residues of an amino acid sequence starting with residue 1 of, and having at least about 95% identity to, the amino acid sequence of SEQ ID NO: 1, 3, 5, 7, 9, 11, 13, 15, 16, or 17. Such polypeptide may stimulate neurite outgrowth or neurogenesis. A nucleic acid may encode the polypeptide. A vector may include the nucleic acid. A host cell, such as an embryonic stem cell, may include the vector.