Predicted tertiary structure of tammar USM1 modelled on human MSMB. (a) The secreted forms of human MSMB and tammar USM1 (predicted) are aligned with the N-terminal domain (above) and the C-terminal domain (below). The ten cysteine residues in MSMB (highlighted in pink with positions indicated) form five disulfide bonds as shown (blue lines). Cysteine residues homologous to C64, C73 and C87 are absent in USM1 due to its C-terminal truncation, leaving C40 in USM1 (homologous to C37 in MSMB) unpaired. We suggest that an additional C-terminal cysteine residue (C59), also conserved in brushtail possum and opossum, instead forms a disulfide bond with C40. (b) The secreted forms of human MSMB (monomer) and tammar USM1 modelled on the crystal structure of human MSMB  (Protein Data Bank accession code 3IX0) using ESyPred3D [65, 66] and displayed as a cartoon with β strands in rainbow colour using Jmol . The approximate positions of disulfide bonds are indicated by pink bars. Predicted disulfide bonds in tammar USM are shown in homologous positions. It is evident that C40 and C59, which lies within the short, flexible C-terminal tail, are in close proximity and likely to form a disulfide bond.