what are cys residues
Cysteine residues in homologous proteins (those of common ancestry) are very conservative according to PAM 250 scoring matrix . Only Trp is statistically more conservative than Cys. Fig 4. PAM 250 scoring matrix of Dayhoff mutation data matrix. Note that the highest values indicating replacement of a residue into itself is for Trp and Cys. That means the two residues are the most conservative in homologous sequences. The reason is that cysteine - because of disulfide bridges - plays very important role in stabilization of protein structure at higher level.