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Hi Ethan, Your effort to play devils advocate is appreciated - I have not seen much debate on the applicability of TLS so at risk of diverging into a separate thread: I would not argue that TLS is necessarily a valid model for correlated motion in proteins because it 'works' in fitting the available Bragg data (or B-factors) which only measure the amplitudes of the atomic displacements. Many different models of correlation might be paramaterized to fit the conventional crystal data and the real benefit of doing the TDS analysis is that it is sensitive to the nature of correlations atomic motions. Suppose all atoms have about the same temperature factor. In this case we get a perfect TLS model with just the T and have the whole protein rigid and moving in perfect correlation. Equally well we might think that all atoms are independent oscilliators in similar microenvironments with there is no correlation at all (this is the Einstein model of a solid!). Seems like the TLS model will always tend to lump motions into an overall T and overestimate this component. This thought experiment can be extended with atoms on the outside having more motion just because they are freer to move and not necessarily indictating that the protein is oscillating about a center (LS). TDS is definately difficult to analyze because for normal globular proteins it tends to look like other 'uninteresting' sources of background scatter and is difficult to separate out (I think Colin made this point). However, I'm not sure that I would agree that the jury is really out on the simple TLS models since there are a lot of decent biophysical probes of correlated motion in proteins that should be able to support this picture if it were correct. These include inelastic neutron scattering experiements (Cusak), Mossbauer experiments with Fe in myoglobin (Parak) and measurements of backbone dynamics on interleukin-1b by NMR (Clore). It seems to me that these studies all indicate that ambient motions in protein are dominated by fast, local, rattling movements. In particular, the measurements of normal mode spectra have the lowest order modes (equivalent to the TLS parameters) as relatively small contributors to the overall motion. Of course I appeciate the attempts to advance the crystallographic refinement models for B-factors beyond the simple local restraints devised by Konnert and Hendrickson over 30 years ago and that we still use almost unchanged! Cheers John