To properly evaluate the present study, it is important to place it in
the context of other large-scale analyses of protein sequences.  The
purpose of this chapter is to provide a brief survey of large-scale
studies which considered all or many of the known protein sequences.
This has been an active research field since the early 90's.  Several
different approaches have been tested.  These studies are mainly
divided into two categories: those focused on finding significant
motifs, patterns and domains within protein sequences, and those which
apply to complete proteins. Another class of studies which use
alternative representations of protein sequences is also discussed.