Hierarchical cluster analysis of serum matrix metalloproteinase levels in patients with ankylosing spondylitis. Results are displayed as a heat map and dendrogram in which the relative levels of matrix metalloproteinases (MMPs) are represented by shades of yellow/red (high) and blue (low). Each row represents the MMP profile for an individual patient, each of which is represented by a number on the vertical axis. Each column represents a different MMP (MMP-1, MMP-2, MMP-3, MMP-8 and MMP-9, left to right). C1, high MMP cluster; C2, low MMP cluster.