Percentages of association of loop clusters (length between 1 and 7) with secondary structures. Frequencies were calculated using the APC rule. The "#' state, which corresponds to missing information in the PDB files (mainly positions for which no electronic density was observed) is also represented and considered with the coil state (c+#). The percentages are indicated for the high redundancy (HR) database (90 %, with the exception of a few clusters for which the chosen redundancy level was lower, Additional file 5), as well as for the 25 % database, when occurences of the considered loop cluster species are sufficient (≥30). Loop clusters are classified, within a given cluster length (L<4, L = 4, L = 5, L = 6 and L = 7), in the decreasing order of the number of residues "0" (this number is indicated near to the coil values, at the top of the figure).