Blocks of multiple sequence alignment for 24 representative mammals were extracted from around the conserved cysteine positions that are discussed in the text. Mus musculus position labeled with a dot. Lower panel shows the location of the cysteine (small yellow circle) in the cartoon tetramers (circles: VS domain, squares: P domain, 1:blue, 2:green, 3:yellow, 4:red). Connecting lines indicate predicted disulfide crosslinks 1 through 4 as numbered in the text. “5” marks an alternative to Disulfide 4, a self-reacting crosslink. Mm4:C233 is strictly conserved but is not outward facing and cannot form a crosslink.