text: The CSI is a graph-based technique that essentially employs an amino acid-specific digital filter to convert every assigned backbone chemical shift value into a simple three-state (-1, 0, +1) index.  This approach generates a more easily understood and much more visually pleasing graph of protein chemical shift values.   In particular, if the upfield Hα chemical shift (relative to an amino acid-specific random coil value) of a certain residue is > 0.1 ppm, then that amino acid residue is assigned a value of -1.  Similarly, if the downfield Hα chemical shift of a certain amino acid residue is > 0.1 ppm then that residue is assigned a value of +1.  If an amino acid residue's chemical shift is not shifted downfield or upfield by a sufficient amount (i.e. <0.1 ppm), it is given a value of 0.  When this 3-state index is plotted as a bar graph over the full length of the protein sequence, simple inspection can allow one to identify beta strands (clusters of +1 values), alpha helices (clusters of -1 values), and random coil segments (clusters of 0 values).  A list of the amino acid-specific random coil chemical shifts for CSI calculations is given in Table 1.  An example of a CSI graph for a small protein is shown in Figure 1 with the arrows located above the black bars indicating locations of the beta strands and the rectangular box indicating the location of a helix.