The crystal structure at 1.54 Å resolution of a double mutant of interleukin-1β (F42W/W120F), a cytokine secreted by macrophages, was determined by multiple-wavelength anomalous dispersion (MAD) using data from highly twinned selenomethionine-modified crystals. The space group is P43, with unit-cell parameters a = b = 53.9, c = 77.4 Å. Self-rotation function analysis and various intensity statistics revealed the presence of merohedral twinning in crystals of both the native (twinning fraction α ≃ 0.35) and SeMet (α ≃ 0.40) forms. Structure determination and refinement are discussed with emphasis on the possible reasons for successful phasing using untreated twinned MAD data.
