Mutagenesis of rHuEPO. Amino acid residues in the rHuEPO sequence (black circles with white letters) and rHuEPO variants (adjacent to rHuEPO sequences with shaded symbols) are indicated. O^c, substitution of a termination codon, TGA. The shape of the symbols indicates the effect of the mutation on RIA-9G8A immunoreactivity. The shading code represents the effect on in vitro bioactivity.