Complex sentence: The human serpin naming system is based upon a phylogenetic analysis of approximately 500 serpins from 2001, with proteins named serpinXY where X is the clade of the protein and Y the number of the protein within that clade.
Split into simpler sentences: The human serpin naming system is based upon a phylogenetic analysis of approximately 500 serpins from 2001. The proteins are named serpinXY where X is the clade of the protein and Y the number of the protein within that clade.