PROSITE is a compilation of sites and patterns found in protein sequences. The use of protein sequence patterns (or motifs) to determine the function of proteins is becoming very rapidly one of the essential tools of sequence analysis. This reality has been recognized by many authors. While there have been a number of recent reports that review published patterns, no attempt had been made until very recently [5,6] to systematically collect biologically significant patterns or to discover new ones. It is for these reasons that we have developed, since 1988, a dictionary of sites and patterns which we call PROSITE. Some of the patterns compiled in PROSITE have been published in the literature, but the majority have been developed,in the last two years, by the author.