The conserved branch length score (Friedman et al., 2009) is the sum of phylogenetic branch lengths between species that contain a site. To help control for individual UTR conservation, 3' UTRs were separated by conservation rate into ten equally sized bins, and a unique set of branch lengths based on 3' UTR sequence alignments was constructed for each bin. Site conservation is defined by conserved branch length, with each site type having a different threshold for conservation: