Structure of Fab 59. Fab 59 (A) is a fully human Fab containing one TPO agonist peptide (blue stripe) in the heavy-chain CDR3 and a second TPO agonist peptide in the light-chain CDR2, which activate the TPO receptor. In contrast, a related Fab, FabX4b (B), containing only one TPO agonist peptide, is not able to activate the TPO receptor until made dimeric by an anti-tag IgG antibody (C) that binds the hemagglutinin (HA) tag (purple ball) on FabX4c. Illustration by Paulette Dennis.