Figure 1: A preference for an even versus odd number of cysteines in proteins from thermophilic archaea. The dataset used for these plots consists of proteins with sizes ranging from 150 to 200 amino acids, the expected trend being more apparent for this class of proteins. (a) Numerous hyperthermophilic and thermophilic archaea show a clear propensity for even numbers of cysteine residues. This trend suggests an abundance of disulfide bonds. Nine examples are shown. (b) Selected nonthermophilic species (all halophiles) are shown as controls. In these cases, the plotted lines are nearly monotonic, indicating an absence of significant disulfide bonding in the halophiles.