Distribution of EC numbers without a single representative sequence in public protein sequence databases (globally orphan EC numbers). The figure is based on 3736 known enzymatic activities. Image is generated using ATT Graphviz software. The EC numbers are organized in a tree hierarchy. Ratio of orphan EC numbers for each EC group is indicated on the edge from its parent node. As shown on the graph, about 1/3 of currently known EC numbers are globally orphan.