Domain structure shows features unique to endogenous TPO.

The amino terminus (first 153 amino acids) of TPO contains 4 conserved cysteine residues and is 17% identical to erythropoietin (EPO; ∼50% identical if neutral substitutions are taken into account). It contains the entire receptor-binding region. The shaded boxes show the predicted α-helical regions of TPO. The carboxyl terminus (amino acids 154 to 332) of the molecule appears to be unique to TPO and contains theN-linked glycosylation sites indicated by solid arrows. Adapted from Foster and Hunt47 with permission.