  It is well known amongst molecular biologists that proteins with a common
ancestor and that perform the same function in similar organisms, can have
rather different amino-acid sequences. Mutations have altered the amino-acid
sequences without affecting the function; this is called neutral evolution. A
simple model of a protein in which the interactions are encoded by sequences of
bits is introduced, and used to study how mutations can change these bits, and
hence the interactions, while maintaining the stability of the protein
solution. This stability is a simple minimal requirement on our model proteins
which mimics part of the requirement on a real protein to be functional. The
properties of our model protein, such as its second virial coefficient, are
found to vary significantly from one model protein to another. It is suggested
that this may also be the case for real proteins in vivo.
