A tier organizes a collection of one or more feature types into a single horizontal row in the view. One of the viewing options is whether the features within that tier are 'collapsed' or 'expanded'. In the collapsed view (top) features lie on top of one another and thus appear along a single line. In this view the EST alignments (shown in light green) are collapsed. In the expanded view (below) the tier disallows overlapping features and thus as many lines as needed are used. The individual EST alignments (one line per EST) now appear separately. The tiers that flank the EST tier, alignments of peptide from other species (orange) or from Drosophila (red), are shown collapsed in both the top and bottom panels.
Lewis et al. Genome Biology 2002 3:research0082.1-0082.14 doi:10.1186/gb-2002-3-12-research0082