CTree has been designed for the quantification of clusters within viral phylogenetic tree topologies. Clusters are stored as individual data structures from which statistical data, such as the Subtype Diversity Ratio (SDR), Subtype Diversity Variance (SDV) and pairwise distances can be extracted. This simplifies the quantification of tree topologies in relation to inter- and intra-cluster diversity. Here the novel features incorporated within CTree, including the implementation of a heuristic algorithm for identifying clusters, are outlined along with the more usual features found within general tree viewing software.