Proc Natl Acad Sci U S A 87, 4576–4579 (1990). Woese, C. R. & Fox, G. E. Phylogenetic structure of the prokaryotic domain: the primary kingdoms. Proc Natl Acad Sci U S A 74, 5088–5090 (1977).

As an agent, your first responsibility is to identify the league for which your particular client (protein) is best suited. There are, of course, the standard prokaryotic, yeast and mammalian leagues.