+Current address: Bragg Institute, Australian Nuclear Science and Technology Organisation, Locked Bag 2001, Kirrawee DC, NSW 2232, Australia.

++Current address: Department of Biological Sciences, University of Essex, Wivenhoe Park, Colchester CO4 3SQ, UK.

1How close to the Wang limit is it possible to go for a successful experiment is a moot point. Using the same APS beamline that we used for S-SAD experiments (described below) that were beset with problems of harmonic contamination (at the time) at wavelengths above 1.8 Å, Dauter and colleagues were able to solve the structure of proteinase K, containing ten S, one Cl and one Ca atoms, using 0.98 Å X-rays with an expected Bijvoet ratio of only 0.46% (Wang et al., 2006BB68).

2Supplementary data for this paper are available from the IUCr electronic archives (Reference: WA5024 ). Services for accessing these data are described at the back of the journal.

3To avoid the unwanted harmonics the Si(220) crystal on ID22 has now been replaced by a Si(111) crystal, where the second-order reflection is absent.

4The second harmonic is present

5The blue Cu nitrite reductase from Alcaligenes xylosoxidans.