Ten cysteine mutations in the Thermobifida fusca cellulase Cel6A and Cel6B catalytic domains were created at different positions by site-directed mutagenesis for the purpose of testing their effects on thermostability and catalytic activity. The level of in vivo expression of these mutant proteins was significantly reduced by many of these mutations. There were positional effects of the mutations, which are probably due to the formation of incorrect disulfide bonds. These results increase our understanding of the role of cysteine residues in protein folding, and also help guide the choice of sites for other mutations in these enzymes.