= Solution
The useful feature is the selenium atom's wavelength-dependent <anomalous X-ray scattering>. Close to its absorption edge the atomic scattering factor is complex:
$$
f_{\rm Se}=f_0+f'(\lambda)+if''(\lambda),\qquad
F(\mathbf h,\lambda)=\sum_j f_j(\lambda)e^{2\pi i\mathbf h\cdot\mathbf r_j}.
$$
Replacing a sulfur-containing side chain by its selenium analogue introduces relatively strong, spatially localized anomalous scatterers. This is the diffraction-physics reason for the label; no biological preparation procedure is needed for the calculation.
The Se substructure can be located from intensity differences or <Patterson maps>. Measurements at several wavelengths change its known complex contribution while leaving most other atoms' scattering nearly unchanged. Interference between that contribution and the unknown remainder constrains the otherwise missing <structure factor> phases. Both wavelength-dependent dispersive differences and differences between <Friedel pairs> contribute to <multiwavelength anomalous diffraction>. Several wavelength constraints resolve phase ambiguities that remain with one intensity difference. The resulting phased Fourier synthesis yields the electron-density map.
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