  We study the impact of mutations (changes in amino acid sequence) on the
thermodynamics of simple protein-like heteropolymers consisting of N monomers,
representing the amino acid sequence. The sequence is designed to fold into its
native conformation on a cubic lattice. It is found that quite a large
fraction, between one half and one third of the substitutions, which we call
'cold errors', make important contributions to the dynamics of the folding
process, increasing folding times typically by a factor of two, the altered
chain still folding into the native structure. Few mutations ('hot errors'),
have quite dramatic effects, leading to protein misfolding. Our analysis
reveals that mutations affect primarily the energetics of the native
conformation and to a much lesser extent the ensemble of unfolded
conformations, corroborating the utility of the ``energy gap'' concept for the
analysis of folding properties of protein-like heteropolymers.
