The proportion of domain families represented by CATH fold groups. Within the CATH database [19, 20], structures are grouped into fold groups on the basis of both overall shape and connectivity of their secondary structures. Domain families are related at the 35% sequence identity level by complete linkage clustering. The number of domain families within each fold group gives a measure of the sequence diversity of that fold group. A group of 54 CATH fold groups (only 6.6% of the cumulative total of CATH fold groups) accounts for 76% of domain families, as shown by the dotted lines.