Disclosed is a novel physiologically active protein, which is a human granulocyte colony stimulating factor isoform, constructed in order to increase the in vivo lifetime of human granulocyte colony sedtimulating factor. The human granulocyte colony stimulating factor isoform comprises a polypeptide and polyethylene glycol (PEG) bound thereto as a non-protein polymer. A specific site of the polypeptide is selected so that polyethylene can be bound to the site while not adversely affecting the activity of the protein. The amino acid of the site is modified with cysteine and polyethylene glycol is bound to the modified site. A pharmaceutical composition comprising the isoforms, genes encoding the isoforms, and a primer for modifying the amino acid sequence are also disclosed.