Loop sequences and sources for all test cases.

For statistics Ncontact, Nout, NSC, and NHB, residues with sequence positions within two residues of each loop residue were excluded from the calculation; only non-hydrogen atoms were considered.
aAverage number of residues that make at least one atom-atom contact (distance <4.0 Å) with each loop residue.
bAvg. number of residues outside the loop that make an atom-atom contact (dist. <4.0 Å) with each loop residue.
cAvg. number of residues that make an atom-atom contact (dist. <4.0 Å) to a loop residue involving an atom requiring side-chain placement (not N, C, Cα, Cβ, O).
dAvg. number of hydrogen bonds per residue, defined as donor/acceptor pairs with distance less than 3.2 Å.
eTest included two crystallographic neighbors that interact with loop [21], [22].
fLoop with irregular structure that remains rigid upon binding to inhibitors or protein partners; see cited references.
gUnpublished 275-residue protein crystal structure from W. Weis and colleagues (see Figs. 5A–D).